PDF20 Spec 14 Tagged PDF

14.8 Tagged PDF

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14.8 Tagged PDF 14.8.1

General

Tagged PDF (PDF 1.4) is a stylised use of PDF that builds on the logical structure framework described in 14.7, "Logical structure". It defines a set of standard structure types and attributes that allow page content (text, graphics and images, as well as annotations and form fields) to be extracted and reused for other purposes. A tagged PDF document is one that conforms to all rules described in all of the subclauses in 14.8, "Tagged PDF". A tagged PDF document shall contain a mark information dictionary (see "Table 353 — Entries in the mark information dictionary") with a value of true for the Marked entry. NOTE

Tagged PDF is intended for use by tools that perform operations such as: •

Extraction of text and graphics for pasting into other applications

•

Automatic reflow of page contents – text as well as associated graphics and images or annotations and form fields – to fit a display area of a different size than was assumed for the original layout

•

Processing of content for such purposes as searching, indexing, and spell-checking

•

Conversion to other common file formats (such as HTML, XML, and RTF) with document structure preserved

•

Making content accessible to users with disabilities

14.8.2

Tagged PDF and page content

14.8.2.1

General

Like all PDF documents, a tagged PDF document consists of a sequence of self-contained pages, each of which is described by one or more page content streams (including any subsidiary streams such as form XObjects), annotations and form fields. Tagged PDF defines some further rules for organising and marking content streams so that additional information may be derived from them: •

Distinguishing between the author’s original content and artifacts of the layout process (see 14.8.2.2, "Real content and Artifacts").

•

Specifying the content order as intended by the content author to guide the layout process if the PDF processor repurposes the page content (see 14.8.2.5, "Page content order and logical content order").

•

Guaranteeing that the logical order of content can be deterministically derived from a

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combination of logical structure and page content (see 14.8.2.5, "Page content order and logical content order"). •

Representing text in a form from which a Unicode representation may be unambiguously derived (see 14.8.2.6, "Unicode mapping in tagged PDF").

•

Representing word breaks unambiguously (see 14.8.2.6.2, "Identifying word breaks").

•

Marking all content with information for making it accessible to users with visual impairments or other disabilities (see 14.9, "Repurposing and accessibility support").

14.8.2.2

Real content and Artifacts

14.8.2.2.1

General

The content in a document may be divided into two classes: •

The real content of a document comprises graphics objects, annotations and form fields representing material intentionally introduced by the document’s author and necessary to understand the content of the document.

•

All other content is considered to be artifacts, whether generated by the PDF writer in the course of pagination, layout, or other mechanical processes or introduced by the document author for decoration or other purposes that are not relevant for understanding the content of the document.

The document’s logical structure encompasses all real content and describes how real content objects relate to one another. Where artifacts are to be included in the structure tree, they shall be included through the Artifact structure element type, and shall not be considered real content. A document’s real content may include graphics objects in the page content stream and subsidiary XObjects and annotations, including widget annotations. NOTE 1

The above paragraph was clarified in this document (2020).

To support PDF processors in providing accessibility to users with disabilities, tagged PDF documents should use the natural language specification (Lang), alternate description (Alt), replacement text (ActualText), and abbreviation expansion text (E) facilities as described in 14.9, "Repurposing and accessibility support". NOTE 2

ISO 14289 (PDF/UA) specifies the use of tagged PDF to accommodate the needs of users with disabilities.

Tagged PDF processors may make various choices about what page content to consider relevant in a tagged PDF document. A text-to-speech engine, for instance, may decide not to speak running heads or page numbers when the page is turned. In general, PDF processors may do any of the following: •

Disregard elements of page content (for example, skip Link annotations) that are not of interest

•

Treat some page elements as terminals that are not to be examined further (for example, to treat an illustration as a unit for repurposing)

•

Substitute an element with its alternative description (see 14.9.3, "Alternate descriptions")

NOTE 3

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Depending on their goals, different PDF processors can make different decisions in this regard. The purpose of tagged PDF is not to prescribe what the PDF processor does, but to provide sufficient declarative and descriptive information to allow it to make appropriate choices about how to process the content. © ISO 2020 – All rights reserved

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14.8.2.2.2

Specification of Artifacts

For tagged PDF files, an artifact should be explicitly distinguished from real content through either of the following methods: •

By enclosing it in a marked-content sequence with the tag Artifact:

EXAMPLE 1 /Artifact BMC EMC

EXAMPLE 2 (marked-content sequence Artifact with a property list entry) /Artifact <> BDC EMC

•

By inclusion in the logical structure tree through the use of the Artifact structure element type (see "Table 375 — standard structure type Artifact").

NOTE 1

The purpose of the Artifact structure element type is to accommodate artifact content in cases that have positional context relative to real content within the structure tree. An example of such content is line numbers.

For artifacts defined using the marked-content sequence method, the form indicated in EXAMPLE 1 shall be used to identify a generic artifact; the form indicated in EXAMPLE 2 shall be used for those artifacts that have an associated property list. "Table 363 — Property list entries for artifacts" shows the properties that may be included in such a property list. Any content that is not included in the structure tree is an artifact even when not enclosed in a markedcontent sequence using the tag Artifact. NOTE 2

The phrase “any content” above refers to all page content as well as annotations.

Table 363 — Property list entries for artifacts Key

Type

Value

Type

name

(Optional) The type of artifact that this property list describes; if present, shall be one of the names Pagination, Layout, Page or Background.

BBox

rectangle

•

Pagination artifacts are ancillary page features such as running heads, folios (page numbers) or Bates Numbering.

•

Layout artifacts are purely cosmetic typographical or design elements such as footnote rules or decorative ornaments.

•

Page artifacts are production aids extraneous to the document itself, such as cut marks and colour bars.

•

Background artifacts can occur from document templates that are often repeated unchanged across many pages and include images, patterns or coloured blocks that either run the entire length and/or width of the page or the entire dimensions of a structural element.

(Optional) An array of four numbers in default user space units giving the coordinates of the left, bottom, right, and top edges, respectively, of the artifact’s bounding box (the rectangle that completely encloses its visible extent).

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Key

Type

Value

Attached

array

(Optional; pagination and full-page background artifacts only) An array of name objects containing one to four of the names Top, Bottom, Left, and Right, specifying the edges of the page, if any, to which the artifact is logically attached. Page edges shall be defined by the page’s crop box (see 14.11.2, "Page boundaries"). The ordering of names within the array is immaterial. Including both Left and Right or both Top and Bottom indicates a full-width or full height artifact, respectively. Use of this entry for background artifacts shall be limited to fullpage artifacts. Background artifacts that are not full-page take their dimensions from their parent structural element. NOTE

Subtype

name

(2020) The Attached key was accidently omitted from the earlier PDF 2.0 specification and was reinstated in this document.

(Optional; PDF 1.7) The subtype of the artifact. This entry should appear only when the Type entry has a value of Pagination. Valid values are Header, Footer, Watermark, PageNum (PDF 2.0), LineNum (PDF 2.0), Redaction (PDF 2.0) and Bates (PDF 2.0). Additional values may be specified for this entry, provided they comply with the naming conventions described in Annex E, "Extending PDF".

Some properties defined elsewhere may also be used as entries in the property list of an artifact, including Alt (see 14.9.3, "Alternate descriptions"), ActualText (see 14.9.4, "Replacement text"), E (see 14.9.5, "Expansion of abbreviations and acronyms") or Lang (see 14.9.2, "Natural language specification"). Where it is necessary to represent the content of an artifact as text, the property ActualText can be used, for instance, to contain the page number for an artifact with a Subtype of PageNum or the Bates number for an artifact of subtype Bates. NOTE 3

14.8.2.3

Bates numbering is used in the legal, medical, and business fields in some countries to place identifying numbers and/or date/time-marks on images and documents. For example, it is added during the discovery stage of preparations for trial or when identifying business receipts. This process provides identification, protection, and automatic consecutive numbering.

Soft hyphens

In tagged PDF, the visible hyphen that is introduced through the incidental division of a word at the end of a line but which would not be present otherwise, may be represented as a soft hyphen, mapped to the Unicode value U+00AD, in one of the ways described in 14.8.2.6, "Unicode mapping in tagged PDF". NOTE 1

The soft hyphen character represented by the Unicode value U+00AD is distinct from an ordinary hard hyphen, whose Unicode value is U+002D.

The writer of a tagged PDF document shall distinguish explicitly between soft and hard hyphens so that a PDF processor can unambiguously determine which type a given character represents.

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NOTE 2

In some languages, the situation is more complicated: there can be multiple characters affected by hyphenation, and hyphenation can change the spelling of words. See the Example in 14.9.4, "Replacement text".

14.8.2.4

Hidden or invisible page content

For a variety of reasons, elements of a document’s real content can be invisible on the page: they can be clipped; their colour can match the background; or they can be obscured by other, overlapping objects. For the purposes of tagged PDF, page content shall be considered to include all graphics objects in their entirety, regardless of whether they are visible when the document is displayed or printed. NOTE

For example, invisible elements can become visible when content is repurposed, or a text-tospeech engine could choose to speak invisible text.

14.8.2.5

Page content order and logical content order

14.8.2.5.1

General

Page content order shall be defined by the sequencing of graphics objects within a page’s content stream. Logical content order – the ordering for semantic purposes – shall be defined by a depth-first traversal of the document’s logical structure hierarchy. The page content order in a tagged PDF should coincide with the logical content order. NOTE 1

Page content order is constrained by the need to render objects in an order that produces the desired visual appearance. Logical content order is constrained by the need to reflect the order of the content as intended by its author. For example, the running text of a page, as encoded in the page’s content stream, can contain places where it is not possible to make the order in which the text progresses match the logical content order.

Content within a single marked-content sequence (see 14.6, "Marked content") shall be in logical content order for that item of content. NOTE 2

NOTE 3

Both in terms of how page content is encoded in a tagged PDF as well as in terms of how a PDF processor can process that content, efficiency can be impacted if the page content order and logical content order sequences do not coincide. In some cases it is not feasible to make the sequences coincide: •

Regarding the sequence of page objects within a given page, page objects can visually overlap in a way that requires reverse ordering.

•

A logical object can extend over more than one PDF page, such as in the case of a headline spanning two pages in a facing pages layout.

•

A page can contain the beginnings of two separate articles, each of which is continued onto a later page of the document. In logical content order terms, the last words of the first article appearing on the page are not followed by the first words of the second article on the same page, but rather by the continuation of the first article on a different page.

Artifacts not contained within an Artifact structure element are not considered part of the logical content order because only structure elements are part of the logical content order.

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14.8.2.5.2

Sequencing of annotations

Annotations associated with a page are not interleaved within the page’s content stream but are listed in the Annots array in its page object (see 7.7.3.3, "Page objects"). The position of an annotation in the logical content order is determined from the document’s logical structure. Both page content (marked-content sequences) and annotations may be treated as content items that are referenced from structure elements (see 14.7.5, "Structure content"). Structure elements of type Annot, Link, or Form (see 14.8.4.7, "Inline level structure types") explicitly specify the association between a marked-content sequence and a corresponding annotation through an object reference dictionary as described in 14.7.5.3, "PDF objects as content items".

14.8.2.5.3

Reverse-order show strings

The marked-content tag ReversedChars informs the PDF processor that show strings within a marked-content sequence contain characters in reverse order. In order for such text to be extracted or read out aloud, the sequence of the characters as found in the show string operator shall be reversed before using them. If the sequence encompasses multiple show strings, only the individual characters within each string shall be reversed. NOTE 1

In writing systems that are read from right to left (such as Arabic or Hebrew), one expects that the glyphs in a font would have their origins at the lower right and their widths (rightward horizontal displacements) specified as negative. For various technical and historical reasons, however, many such fonts follow the same conventions as those designed for Western writing systems, with glyph origins at the lower left and positive widths, as shown in "Figure 54 — Glyph metrics". Consequently, showing text in such right-to-left writing systems requires either positioning each glyph individually (which is tedious and costly) or representing text with show strings (see 9.2, "Organisation and use of fonts") whose character codes are given in reverse order.

The show strings in a ReversedChars block may have a SPACE (U+0020) character or other whitespace characters at the beginning or end to indicate a word break (see 14.8.2.6.2, "Identifying word breaks") but shall not contain interior SPACE characters or other white-space characters. NOTE 2

EXAMPLE

This limitation is not serious, since a SPACE or other white-space character typically provides an opportunity to realign the typography without visible effect, and it serves the valuable purpose of limiting the scope of reversals for word-processing interactive PDF processors. The sequence

/ReversedChars BMC ( olleH) Tj -200 0 Td ( .dlrow) Tj EMC

represents the text Hello world.

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14.8.2.6

Unicode mapping in tagged PDF

14.8.2.6.1

General

PDF documents conforming with 14.8, "Tagged PDF" should map every character code in any of the content streams or appearance streams of a document to a corresponding Unicode value. Every character code that belongs to a structure element in the structure tree shall map to Unicode, except where an associated Alt or ActualText entry applies to the content to which the character code belongs. NOTE 1

These Unicode values can then be used for such operations as copy-and-paste, searching, text-tospeech conversion, and exporting to other applications or file formats.

NOTE 2

Unicode defines scalar values for most of the characters used in the world’s languages and writing systems, as well as providing a private use area for application-specific characters. Information about Unicode can be found in the Unicode Standard.

The methods for mapping a character code to a Unicode value are described in 9.10.2, "Mapping character codes to Unicode values". Private Use Area Unicode values should only be used if no other Unicode value is available, as no predefined meaning is associated with the Unicode values in the Private Use Area. NOTE 3

An Alt, ActualText, or E entry specified in a structure element dictionary or a marked-content property list (see 14.9.3, "Alternate descriptions" 14.9.4, "Replacement text" and 14.9.5, "Expansion of abbreviations and acronyms") can affect the character stream used by PDF processors. For example, some PDF processors could choose to use the Alt or ActualText entry and ignore all text and other content associated with the structure element and its descendants.

In some cases a required character may not be available in a given font, for example the soft hyphen character. Such a character may be represented either by adding it to the font’s encoding or CMap and using ToUnicode to map it to the appropriate Unicode value, or by using an ActualText entry in the associated structure element to provide substitute characters.

14.8.2.6.2

Identifying word breaks

A document’s text content defines not only the characters in a page’s text but also the words. Unlike a character, which is defined unambiguously, a word is defined by script and context. A repurposing tool needs to determine where it can break the running text into lines; a text-to-speech engine needs to identify the words to be vocalised; spelling checkers and other applications have varying definitions for what constitutes a word. It is not important for a tagged PDF document to identify the words within the text stream according to a single, unambiguous definition that satisfies all of these clients. What is important is that there be enough information available for each client to make that determination for itself. A PDF processor of a tagged PDF document may find words by sequentially examining the Unicode character stream, as augmented by replacement text specified with ActualText (see 14.9.4, "Replacement text"). For this purpose any white-space characters that would be present to separate words in a pure text representation shall be present in the tagged PDF representation of the text.

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NOTE 1

As a consequence, the PDF processor can determine word breaks without having to rely on heuristics based on information such as glyph positioning on the page, font changes, or glyph sizes.

The identification of what constitutes a word shall be unrelated to how the text happens to be grouped into show strings. The division into show strings shall have no semantic significance. In particular, a SPACE (U+0020) or other word-breaking character shall be present in a character stream even if a word break happens to fall at the end of a show string. NOTE 2

Some PDF processors identify words by simply separating them at every SPACE character. Others can be slightly more sophisticated and treat punctuation marks such as hyphens or Em dashes as word separators as well. Still others can identify line-break opportunities by using an algorithm similar to the one in Unicode Standard Annex #29, Unicode Text Segmentation.

14.8.3

Basic layout model

14.8.3.1

General

Tagged PDF’s standard structure types and attributes can be interpreted in the context of a basic layout model that describes the arrangement of structure elements on the page. This model is designed to capture the general intent of the document’s underlying structure and does not necessarily correspond to the one actually used for page layout by the application creating the document (the PDF content stream specifies the exact appearance). NOTE 1

The goal is to provide sufficient information for PDF processors to make their own layout decisions while preserving the authoring application’s intent as closely as their own layout models allow.

NOTE 2

The tagged PDF layout model resembles the ones used in markup languages such as HTML, XML, and RTF, but does not correspond exactly to any of them. The model is deliberately defined loosely to allow reasonable latitude in the interpretation of structure elements and attributes when converting to other document formats. Some degree of variation in the resulting layout from one format to another is to be expected.

14.8.3.2

Reference area

The basic layout model begins with the notion of a reference area. This is a rectangular region used as a frame or guide in which to place the document’s content. Some of the standard structure attributes, such as StartIndent and EndIndent (see 14.8.5.4.3, "Layout Attributes for BLSEs"), shall be measured from the boundaries of the reference area. Reference areas are not specified explicitly but are inferred from context. Those of interest are generally the column area or areas in a general text layout, the outer bounding box of a table and those of its component cells, and the bounding box of a Figure, Form or Formula, or other floating element. The standard structure types can be divided into four main categories according to the roles they play in page layout: •

Grouping elements (see 14.8.4.4, "Grouping level structure types") group other elements into sequences or hierarchies but hold no content directly and have no direct effect on layout.

•

Block-level structure elements (BLSEs) (see 14.8.4.5, "Block level structure types") describe the overall layout of content on the page, proceeding in the block-progression direction.

•

Inline-level structure elements (ILSEs) (see 14.8.4.7, "Inline level structure types") describe the layout of content within a BLSE, proceeding in the inline-progression direction.

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•

Some elements can be grouping elements, BLSEs or ILSEs, depending on context or usage. For example, a Figure structure element can be treated as either a grouping element, a BLSE or an ILSE.

14.8.3.3

Progression direction

The meaning of the terms block-progression direction and inline-progression direction depends on the writing system in use, as specified by the standard attribute WritingMode (see 14.8.5.4.2, "General Layout Attributes"). In Western writing systems, the block direction is from top to bottom and the inline direction is from left to right. Other writing systems use different directions for laying out content. Because the progression directions can vary depending on the writing system, edges of areas and directions on the page are identified by terms that are neutral with respect to the progression order rather than by familiar terms such as up, down, left, and right. Block layout proceeds from before to after, inline from start to end. Thus, for example, in Western writing systems, the before and after edges of a reference area are at the top and bottom, respectively, and the start and end edges are at the left and right. Another term, shift direction (the direction of shift for a superscript), refers to the direction opposite that for block progression — that is, from after to before (in Western writing systems, from bottom to top). BLSEs shall be stacked within a reference area in block-progression order. In general, the first BLSE shall be placed against the before edge of the reference area. Subsequent BLSEs shall be stacked against preceding ones, progressing toward the after edge, until no more BLSEs fit in the reference area. If the overflowing BLSE allows itself to be split — such as a paragraph that can be split between lines of text — a portion of it may be included in the current reference area and the remainder carried over to a subsequent reference area (either elsewhere on the same page or on another page of the document). Once the amount of content that fits in a reference area is determined, the placements of the individual BLSEs may be adjusted to bias the placement toward the before edge, the middle, or the after edge of the reference area, or the spacing within or between BLSEs may be adjusted to fill the full extent of the reference area. BLSEs may be nested, with child BLSEs stacked within a parent BLSE in the same manner as BLSEs within a reference area. Except in a few instances noted (the BlockAlign and InlineAlign elements), such nesting of BLSEs does not result in the nesting of reference areas; a single reference area prevails for all levels of nested BLSEs. Within a BLSE, child ILSEs shall be packed into lines. Direct content items — those that are immediate children of a BLSE rather than contained within a child ILSE — shall be implicitly treated as ILSEs for packing purposes. Each line shall be treated as a synthesized BLSE and shall be stacked within the parent BLSE. Lines may be intermingled with other BLSEs within the parent area. This line-building process is analogous to the stacking of BLSEs within a reference area, except that it proceeds in the inline-progression rather than the block-progression direction: a line shall be packed with ILSEs beginning at the start edge of the containing BLSE and continuing until the end edge shall be reached and the line is full. The overflowing ILSE may allow itself to be broken at linguistically determined or explicitly marked break points (such as hyphenation points within a word), and the remaining fragment shall be carried over to the next line.

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Certain values of an element’s Placement attribute remove the element from the normal stacking or packing process and allow it instead to float to a specified edge of the enclosing reference area or parent BLSE; see "General Layout Attributes" in 14.8.5.4, "Layout Attributes" for further discussion. Two enclosing rectangles shall be associated with each BLSE and ILSE (including direct content items that are treated implicitly as ILSEs): •

The content rectangle shall be derived from the shape of the enclosed content and defines the bounds used for the layout of any included child elements.

•

The allocation rectangle includes any additional borders or spacing surrounding the element, affecting how it shall be positioned with respect to adjacent elements and the enclosing content rectangle or reference area.

The definitions of these rectangles shall be determined by layout attributes associated with the structure element; see 14.8.5.4.5, "Content and Allocation Rectangles" for further discussion.

14.8.4

Standard structure types

14.8.4.1

General

Standard structure types are divided into categories: •

"Document level structure types" identify a whole document or a fragment of a document. In most cases a tagged PDF will contain exactly one document, but in some other cases a tagged PDF contains several documents that have been merged together into one PDF file, or the tagged PDF is a document containing document fragments which have been inserted into the document.

•

"Grouping level structure types" make it possible to organise the overall structure of content in a tagged PDF. For example, a book typically consists of chapters, or a newspaper page consists of several parts called articles, and a scientific publication typically contains sections and several levels of sub-sections.

•

"Block level structure types" are structure types that enclose actual content, like a heading or paragraph.

•

"Inline level structure types" are structure types that enable structural organisation of content inside block level elements and inside specialised structure elements used as block level elements.

Some structure types — for example Table or Figure — may be used as block level types or as inline types, whereas others (e.g., H1) may only be used as block level types. Whenever a provision in clause 14.8, "Tagged PDF", refers to block level types, all structure elements used as block level elements shall be included. Whenever a provision in clause 14.8, "Tagged PDF", refers to inline level types, all structure elements used as inline level elements shall be included. To determine the category that is applicable to a structure element that may either be a block level structure element or an inline level structure element, the following applies: •

If the structure element is used inside a block level element, it is an inline level structure element

•

In all other cases it is a block level structure element.

All structure elements occurring within a tagged PDF document shall have a type matching one of those defined as a Standard Structure Type, or a role map providing a mapping from the non-standard type to a Standard Structure Type. 754

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14.8.4.2

Nesting of standard structure elements

Annex L, "Parent-child relationships between the standard structure elements in the standard structure namespace for PDF 2.0" defines the nesting rules for standard structure elements. In addition, supplemental rules apply for some standard structure elements. These supplemental rules are defined in the subclauses specific to each of the standard structure types. Structure elements other than the standard structure elements identified in clause 14.8.4 "Standard structure types" or in the standard structure namespaces (see 14.8.6.2, "Role maps and namespaces") shall nest in relation to standard structure elements according to the requirements of the structure elements to which they are role mapped. NOTE

14.8.4.3

The nesting rules detailed in Annex L, "Parent-child relationships between the standard structure elements in the standard structure namespace for PDF 2.0" address in detail the following system model: o

Structure elements can be empty.

o

If a structure element contains neither structure elements nor real content it is empty.

o

Unless specifically constrained in 14.8, "Tagged PDF", any number of structure elements can exist in any order inside a parent structure element.

o

Real content is contained inside block level or inline level structure elements.

o

Document level structure elements that are not empty contain other document level, grouping or block level elements, and cannot contain inline level structure elements or content.

o

Grouping structure elements that are not empty contain other document level, grouping or block level elements, and cannot contain inline level structure elements or content.

o

Inline level elements can contain other inline level structure elements but cannot contain any other type of structure element.

Document level structure types

In a tagged PDF file a logical document is a portion of content typically perceived as a semantically self contained document of any granularity. Examples range from short memos to articles, presentations or fillable forms to comprehensive publications like magazines or books. A logical document fragment is a portion of content that constitutes – or is intended or perceived as – just a part of a logical document, regardless of whether it was extracted from a logical document or originated as a document fragment. A tagged PDF file may consist of one or more logical documents or logical document fragments as described in "Table 364 — Document level structure types".

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Table 364 — Document level structure types Structure Type

Category

Description

Document

Encloses a logical document.

DocumentFragment

Document

EXAMPLE 1

A mail merge PDF typically contains a number of letters to different recipients. This implies that the PDF at the top level is one document, containing multiple documents at its child level where each such document is a letter to a recipient.

EXAMPLE 2

A Proceedings PDF generally includes individual papers given at a conference. This implies that the PDF at the top level is one document containing several documents.

(PDF 2.0) Encloses a logical document fragment. A document fragment is typically created by extracting it as a part from an original complete document. As a consequence the structure of the fragment may be incomplete, and content may start at an arbitrary point within the original logical content structure. EXAMPLE 3

NOTE 1

The following are just two possible types of document fragments: •

an extract from an original document is inserted into a new containing document

•

extracts from original documents concatenated into a new document

are

A logical document can contain any number of DocumentFragment elements, including nested logical document fragments.

Each logical document fragment shall define its own logical structure. The logical structure of a logical document fragment may start in the middle of what was the parent document’s logical structure. For example, it could start at an arbitrary paragraph or at a heading level 2 or 3 instead of heading level 1 or even in the middle of a list. NOTE 2

DocumentFragments are especially useful when hierarchical aspects of logical structure from the original document are to be maintained but are incomplete and thus would be difficult to understand or process.

Within each Document or DocumentFragment structure element, all heading structure elements shall either be Hn or H. See "Table 366 — Block level structure types" for more information on heading structure elements. An XMP metadata stream (see 14.3.2, "Metadata streams") in a Document or DocumentFragment structure element may be used to include document metadata for a logical document nested inside a tagged PDF.

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14.8.4.4

Grouping level structure types

This subclause describes structure types for grouping elements, as detailed in "Table 365 — Grouping level structure types", that are used to organise the overall structure of content in a tagged PDF. Table 365 — Grouping level structure types Structure Type

Category

Description

Part

Grouping

Encloses a grouping of structure elements without consideration for their hierarchy. NOTE 1 Part is the semantic equivalent of Div.

A structure element with the type of Part shall inherit the containment requirements and limitations of its parent element. Where the parent element is itself a structure element of type Part, then the inheritance shall recurse to the first parent element whose type is not Part. EXAMPLE 1

Sect

Grouping

•

frontmatter or backmatter in a book

•

a table of contents or bibliography section in a document

•

main body of content in a document

•

advertising section in a magazine

•

a group of pages (for example, a spread in a magazine)

•

a group of form fields

•

a group of Figure standard structure elements

•

a publisher’s indicia

Encloses a grouping of structure elements with consideration for their hierarchy. EXAMPLE 2

The following are just a few of the possible types of content that could be marked as Part:

The following are just a few of the possible types of content that could be marked as Sect: •

clauses in a technical document

•

components of an article in a newspaper or magazine

•

Elements of a recipe “ingredients”, etc.)

(e.g.,

“name”,

“instructions”,

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Structure Type

Category

Description

Div

Grouping

Encloses content structured in fashion that is orthogonal to the semantic structure. It can be used as a role mapping target for custom tags for which no suitable standard structure element is available, or where attributes are applied in a non-semantic fashion. NOTE 2 Nesting Div structure elements allows content to be subdivided.

A structure element with the type of Div shall inherit the containment requirements and limitations of its parent element. Where the parent element is itself a structure element of type Div, then the inheritance shall recurse to the first parent element whose type is not Div. NOTE 3 Div does not change the semantic level of the structure hierarchy. EXAMPLE 3

The following is just one of the possible types of content that could be marked as Div: •

Aside

Grouping

(PDF 2.0) Encloses content that is distinct from other content within its parent structure element. EXAMPLE 4

NonStruct

NOTE

14.8.4.5

Grouping

associating a language attribute to group of content items without implying any semantic aspects

The following are just a few of the possible types of content that could be marked as Aside: •

Callout elements

•

Sidebar elements

•

Commentary accompanying an article

•

Background information for a section in a text book

(Non-structural element) A grouping element having no inherent structural significance; it serves solely for grouping purposes. This type of element differs from a division (structure type Div) in that it should not be interpreted or exported to other document formats. Its descendants shall be processed normally. A structure element with the type of NonStruct shall inherit the containment requirements and limitations of its parent element. Where the parent element is itself a structure element of type NonStruct, then the inheritance shall recurse to the first parent element whose type is not NonStruct.

(2020) This document updated the definition of Part and returned Sect and NonStruct to the above table with revised definitions.

Block level structure types

This subclause describes structure types for paragraph-like elements, as detailed in "Table 366 — Block level structure types", that consist of running text and other content laid out in the form of conventional paragraphs (as opposed to more specialised types of content such as lists and tables).

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Table 366 — Block level structure types Structure Type

Category

Description

P

Block

(Paragraph) A low-level division of content. Although in many cases it will be used for paragraphs it may enclose any low-level division of content. EXAMPLE

The following is just one of the possible types of content that could be marked as P: •

Hn

Block

A paragraph in a newspaper article or a paragraph in the chapter of a novel.

(With n being a sequence of digits representing an unsigned integer greater than or equal to 1) Encloses a low-level division of content usually referred to as a heading. NOTE 1 Low-level content would typically be perceived as a sub-section of a document, whether a section heading, chapter or any other identifiable subdivision of content within a logical document. See the Title element regarding high-level division of content.

Any such heading structure element shall always consist of the uppercase letter "H" and one or more digits, representing an unsigned integer greater than or equal to 1, without leading zeroes or any other prefix or postfix. The heading level is indicated through the chosen structure element type, for example H1 indicates a heading at the topmost level within a document, H2 a heading at the second level, and so forth. NOTE 2 This implies that H7 can be used to indicate a heading on the seventh level, whereas h7, H07, H-7 or H_7 cannot be used as heading structure elements.

The heading level indicated by a heading should reflect the heading level of the tagged content. NOTE 3 The Lbl structure element can be used to enclose section enumeration content or its functional equivalent present inside the heading. EXAMPLE

A section heading in a text book or newspaper article is one example where a heading level would be indicated.

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Structure Type

Category

Description

H

Block

Encloses a low-level division of content usually referred to as a heading. The heading level is not indicated through this structure element, but instead is derived from the nesting of the logical structure within a given Document or DocumentFragment structure element. The H heading structure elements should always be the first structure element within its parent structure element. The H heading structure element shall always be the only heading structure element within its parent structure element. NOTE 1 This implies that within a given logical document it is not acceptable that an H heading structure element is used if a H1, H2, H3 etc. heading structure element is also used inside the same DocumentFragment structure element. NOTE 2 The Lbl structure element can be used to enclose a section number or similar present inside the heading.

Title

Grouping or Block

EXAMPLE 1

As the use of the H heading structure element requires strict document structuring it is typically used only for machine generated documents, documents created from a well structured content data set or documents whose creation is fully controlled by a program such that its structure is strictly guaranteed.

EXAMPLE 2

Scientific publications and technical specifications often follow very strict structuring rules and thus are suitable candidates for use of the H heading structure element.

(PDF 2.0) Encloses content that is usually referred to as the title of a document or high-level division of content. NOTE 3 High-level content would typically be perceived as the title of an article, section, chapter or any other identifiable top-level subdivision of content within a logical document. See the Hn and H elements regarding low-level division of content.

It should occur only once inside the parent grouping structure element, and it should occur at or near the beginning of the content inside that grouping structure element. EXAMPLE 3

FENote

Grouping, Block or Inline

The title of a book, brochure or leaflet are some types of content that can be marked with the Title structure element.

(PDF 2.0) Used to markup footnotes and endnotes. Footnotes and endnotes are content that is not normally read as part of the enclosing content from which it is referenced, but rather consulted at the reading person’s discretion. In order for text to be considered a footnote or endnote, there should be a reference from the enclosing content to the footnote or endnote. Such reference may be achieved by means of a Link structure element through a structure destination in its link annotation (see "Table 368 — General inline level structure types"), or use of Ref in structure elements (see "Table 355 — Entries in a structure element dictionary"). NOTE 4 Text that is labelled as a note – like this paragraph –but is intended to be normally read together with the enclosing content is not a footnote or endnote.

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14.8.4.6

Sub-block level structure type

"Table 367 — Sub-block level structure type" defines a structure type similar to block level types but intended for use in an inline context. An example of this usage is provided in H.8.4, "Example of Sub standard structure type". Table 367 — Sub-block level structure type Structure Type

Category

Description

Sub

Inline

(PDF 2.0) (Sub-division of a block level element) Encloses content typically perceived as a sub-division inside a block level structure element. A Lbl structure element may be used inside a Sub structure element to enclose a line number, verse number or similar, if present. If a Sub structure element is used, all other content inside the same block level element that is the parent of the Sub structure element, should also be enclosed in Sub structure elements. Examples: •

a verse in a poem or sacred scripture

•

a line-numbered document

•

a line of source code

•

a line in a postal address

14.8.4.7

Inline level structure types

14.8.4.7.1

General

Unless otherwise noted in 14.8.4.2, “Nesting of standard structure elements”, any inline structure element is optional. It may occur once or more than once inside its parent structure element. The other children of its parent structure element, if any are present, may be other inline structure elements or actual content. Inline structure elements and portions of actual content inside a parent structure element may occur in any combination and in any order. Unless restricted by their type, structure elements and portions of actual content inside a parent structure element may occur in any combination and in any order.

14.8.4.7.2

General inline level structure types

"Table 368 — General inline level structure types" defines standard structure types for inline level structure types.

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Table 368 — General inline level structure types Structure Type

Category

Description

Lbl

Inline

(Label) Encloses content that distinguishes it from other content inside the same parent element. In a list item (see 14.8.4, "Standard structure types") the Lbl structure element may enclose the bullet for list items in unordered lists or digits and characters used for numbering of list items in ordered lists. For headings it may be the number of the chapter. For a definition list it may enclose the term to be defined. For key value pairs it may enclose the key for which a value is provided. For an entry in a table of contents there may be two Lbl structure elements, one for a chapter number, and one for the page number at which the chapter starts, whereas the actual text for the chapter heading is the remaining portion of the table of contents list item. EXAMPLE 1

Span

762

Inline

The following are just a few of the possible types of content that could be marked as Lbl: •

bullets for list items in an unordered list

•

digits or characters used for numbering list items in an ordered list

•

the number in the Caption for a Figure or Table

•

in a form, the label of a form field

•

in footnotes, the number or symbol matching the reference from the text

•

in headings, the enumeration of the headings

•

in a dictionary, the word for which a translation is provided

•

in a definition list, the term for which a definition is provided

•

in key value pairs, the key for which a value is provided

•

in a question and answer sequence in an interview, visual cues for designating questions and answer

A generic inline portion of content having no particular inherent characteristics. It can be used, for example, to delimit a range of text with certain attributes. EXAMPLE 4

A word inside a sentence is in a different language than the surrounding text, and is contained in a Span with a Lang attribute indicating the applicable language.

EXAMPLE 5

A custom set of structure element types defines custom inline structure elements which are mapped to the Span structure element in the RoleMap to enable a PDF processor unaware of the custom set of structure element types to essentially ignore the structure element.

ISO 32000-2:2020(E)

Structure Type

Category

Description

Em

Inline

(PDF 2.0) (Emphasis) Encloses content for the purpose of emphasis. The level of stress that a particular piece of content has is given by its number of ancestor Em structure elements. The placement of stress emphasis changes the meaning of the sentence. The element thus forms an integral part of the content. The precise way in which stress is used in this way depends on the language. EXAMPLE 2

These examples show how changing the stress emphasis changes the meaning. First, a general statement of fact, with no stress:

Cats are cute animals.

By emphasizing the first word, the statement implies that the kind of animal under discussion is in question (maybe someone is asserting that dogs are cute):

Cats are cute animals.

By moving it to the adjective, the exact nature of the cats is reasserted (maybe someone suggested cats were mean animals):

Cats are cute animals.

Strong

Inline

(PDF 2.0) Encloses content for the purpose of strong importance, seriousness or urgency for its contents. EXAMPLE 3

In this example the Strong element is used to denote the content that the user is intended to read first:

Your tasks for today:

Turn off the oven.

Put out the trash.

Do the laundry.

Link

Grouping, An association between content enclosed by the Link structure element and a Block or corresponding link annotation (see 12.5.6.5, "Link annotations"). Inline

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Structure Type

Category

Description

Annot

Grouping, Either an association between the content enclosed by the Annot structure Block or element and one or more corresponding PDF annotations (see 12.5, Inline "Annotations"), or a mechanism to include one or more PDF annotations in the structure tree. If more than one annotation is referenced in an Annot structure element, they shall be of the same annotation type. Annot shall not be used for link annotations (see the Link structure element) or widget annotations (see the Form structure element). All other annotation types may be referenced by this structure element. EXAMPLE 6

Form

NOTE 1

The following are just a couple of the possible types of content that could be marked as Annot: •

Markup annotations indicating change requests like requests for deletions, modifications or insertions.

•

Highlighting of certain content.

Grouping, Either an association between content enclosed by the Form structure element and Block or a corresponding widget annotation or a mechanism to include a widget annotation Inline in the structure tree. In a tagged PDF, Form shall be used for each PDF widget annotation that belongs to the real content of the document. NOTE

Form structure elements often include Lbl structure elements to mark up a form field’s label (if any).

EXAMPLE 7

The following are the possible types of content that could be marked as Form: •

Form fields in a PDF containing a fillable form would be marked as Form structure element.

•

Non-interactive forms, that is, content enclosed in a structure element with the PrintField attribute, would be marked with Form structure elements.

(2020) This document redefined available categories for Link and Annot structure elements types.

Tagged PDF link elements (standard structure type Link) use PDF’s logical structure facilities to establish the association between content items and link annotations, providing functionality comparable to HTML hypertext links. The following items may be children of a link element: •

One or more content items or other ILSEs (except other links) if A, Dest and PA keys of all of them have identical values

•

One object reference (see 14.7.5.3, "PDF objects as content items") to one link annotation associated with the content

NOTE 2

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An SD entry in the GoTo or GoToR action in a Link annotation facilitates linking directly to a target structure element as opposed to just targeting an area on a page.

ISO 32000-2:2020(E)

14.8.4.7.3

Ruby and warichu elements

Ruby text is a side note, written in a smaller text size and placed adjacent to the base text to which it refers. It is used in Japanese and Chinese to describe the pronunciation of unusual words or to describe such items as abbreviations and logos. Warichu text is a comment or annotation, written in a smaller text size and formatted onto two smaller lines within the height of the containing text line and placed following (inline) the base text to which it refers. It is used in Japanese for descriptive comments and for ruby annotation text that is too long to be aesthetically formatted as a ruby. "Table 369 — Ruby and Warichu structure types" defines standard structure types for Ruby and warichu text. Table 369 — Ruby and Warichu structure types Structure Type

Category

Description

Ruby

Inline

Structure element that wraps around an entire ruby assembly. It shall contain one RB element followed by either an RT element or a three-element sequence consisting of RP, RT, and RP. Ruby structure elements and their content elements shall not break across multiple lines.

RB

Inline

(Ruby base text) The full-size text to which the ruby annotation is applied. RB may contain text, other inline elements, or a mixture of both. It may have the RubyAlign attribute.

RT

Inline

(Ruby annotation text) The smaller-size text that shall be placed adjacent to the ruby base text. It may contain text, other inline elements, or a mixture of both. It may have the RubyAlign and RubyPosition attributes.

RP

Inline

(Ruby punctuation) Punctuation surrounding the ruby annotation text. It is used only when a ruby annotation cannot be properly formatted in a ruby style and instead is formatted as a normal comment, or when it is formatted as a warichu. It contains text (usually a single LEFT PARENTHESIS or RIGHT PARENTHESIS or similar bracketing character).

Warichu

Inline

(Warichu) The wrapper around the entire warichu assembly. It shall contain a three-element sequence consisting of WP, WT, and WP. Warichu structure elements (and their child structure elements) may wrap across multiple lines, according to the warichu breaking rules described in the Japanese Industrial Standard (JIS) X 4051-2004.

WT

Inline

(Warichu text) The smaller-size text of a warichu comment that is formatted into two lines and placed between surrounding WP elements.

WP

Inline

(Warichu punctuation) The punctuation that surrounds the content in the WT structure element. It typically contains text (usually a single LEFT PARENTHESIS or RIGHT PARENTHESIS or similar bracketing character). According to JIS X 4051-2004, the parentheses surrounding a warichu may be converted to a SPACE (nominally 1/4 EM in width) at the discretion of the formatter.

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14.8.4.8

Other structure types

14.8.4.8.1

General

There are a number of other structure types that have a distinct internal structure on their own, or that may be used as grouping level structure elements, as block level structure elements or as inline level structure elements, depending on the context in which they are used.

14.8.4.8.2

Standard structure types for lists

This clause describes structure types for organising the content of lists. H.8.3, "Hierarchical lists" provides an example of hierarchical list entries. "Table 370 — List standard structure types" defines standard structure types for lists. Table 370 — List standard structure types Structure Type

Category

Description

L

Block or Inline

(List) Encloses content consisting of a sequence of items that are semantically related with each other. If a Caption is present, it shall be either the first or last child in the L (list) structure element. The ListNumbering attribute (see 14.8.5.5, "List attribute") may be used to indicate the type of ordered or unordered list. The ContinuedList and ContinuedFrom attributes (see 14.8.5.5, "List attributes") may be used to indicate relationships with other L (list) structure elements. EXAMPLE 1

LI

Internal to L (List) structure elements

(List Item) Encloses content for an individual member of a list. Children (see Annex L, "Parent-child relationships between the standard structure elements in the standard structure namespace for PDF 2.0") of the list item may occur in any order or combination. NOTE

LBody

Internal to LI (List item) structure elements

Bulleted lists, numbered lists, tables of contents, indexes, dictionaries, and lists of key value pairs are all examples of content that would use the L structure element.

LI structure elements often include Lbl structure elements (see Table 368 — General inline level structure types) to mark up a list item's label (if any).

(List Item Body) Encloses the actual content of a list item. EXAMPLE 2

In a dictionary list the list item body contains the translation or definition of the term.

To represent hierarchical lists, the child L (list) structure elements shall be a direct child of its parent L (list) structure element or contained within a Div structure element belonging to the parent L (list). 766

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NOTE

Lists can occur within the content of an LBody structure element. Such lists are not considered part of the hierarchy.

14.8.4.8.3

Table structure types

"Table 371 — Table standard structure types" defines standard structure types for tables. Table 371 — Table standard structure types Structure Type

Category

Description

Table

Block

A two-dimensional logical structure of cells, possibly including a complex substructure. If a Caption is present, it shall be either the first or last child of the Table structure element.

TR

Internal to a A row of table header cells (TH) or table data cells (TD) in a table. Table structure

TH

Internal to a A table header cell containing content describing one or more rows, Table columns or rows and columns of the table. structure The following attributes can be used with the TH structure element:

TD

•

RowSpan

•

ColSpan

•

Headers

•

Scope

•

Short

Internal to a A table cell containing content that is part of the table’s content. Table The following attributes can be used with the TD structure element: structure •

RowSpan

•

ColSpan

•

Headers

THead

Internal to a (Optional) A group of TR structure elements that constitute the header of Table a table. structure The THead structure element is optional insofar as when rows of header cells are present they may, but are not required to be, so enclosed.

TBody

Internal to a (Optional) A group of TR structure elements that constitute the main body Table portion of a table. structure The TBody structure element is optional insofar as when rows of cells are present they may, but are not required to be, so enclosed.

TFoot

Internal to a (Optional) A group of TR structure elements that constitute the footer of a Table table. structure The TFoot structure element is optional insofar as when rows of cells belonging to footer row(s) are present they may, but are not required to be, so enclosed.

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If the Headers attribute (see 14.8.5, "Standard structure attributes") is not specified, any cell in a table may have multiple headers associated with it. These headers are defined either explicitly by the Headers attribute, or implicitly, by the following algorithm: To find headers for any data or header cell, begin from the current cell position and use the current value of WritingMode to search towards the first cell in the appropriate horizontal/vertical direction. The search terminates when any of these conditions is reached: •

the edge of the table is reached

•

a data cell is found after a header cell

•

a header cell has the Headers attribute set — the headers that are specified are appended to the row/ column list that is being built

When a header cell is found in the search and the (implicit or explicit) Scope attribute of the header cell is either Both or Row/Column, the header cell is appended to the end of the list of row/column headers, resulting in a list of headers ordered from most specific to most general. NOTE

This algorithm works for languages with different intrinsic directionality of the script (such as right-to-left) because the structure always reflects the logical content order of the table.

14.8.4.8.4

Caption structure type

The standard structure type Caption, defined in "Table 372 — Standard structure type Caption", encloses content that serves as a caption for tables, lists, images, formulas, media objects or other types of content. Table 372 — Standard structure type Caption Structure Type

Category

Description

Caption

Grouping or Block

For lists and tables a Caption structure element may be used as defined for the L (list) and Table structure elements. In addition a Caption may be used for a structure element or several structure elements. A structure element is understood to be "captioned" when a Caption structure element exists as an immediate child of that structure element. The Caption shall be the first or the last structure element inside its parent structure element. The number of captions cannot exceed 1. While captions are often used with figures or formulas, they may be associated with any type of content. NOTE

In principle, captions can appear in a nested fashion. For example, several smaller images belonging to a group of images can each be accompanied by a caption, and the group of these images as a whole is accompanied by a caption as well.

EXAMPLE

768

The following are just a few of the possible types of content that could be marked as Caption: •

Caption for an image, list, table, or formula

•

Caption for a group of images or a sequence of graphics

•

Group of images with a caption, plus a caption for each of the images inside the group of images

ISO 32000-2:2020(E)

14.8.4.8.5

Figure structure type

"Table 373 — Standard structure type Figure" defines the Figure standard structure type. The standard structure type Figure encloses content that represents one or more complete graphics objects. It shall not appear between the BT and ET operators delimiting a text object (see 9.4, "Text objects"). A Figure element may have logical substructure, including other Figure elements. For repurposing purposes it may be treated as visually static, without examining its internal contents. It should have a BBox attribute (see 14.8.5, "Standard structure attributes"). For repurposing and accessibility purposes, a Figure element should have either an Alt entry or an ActualText entry in its structure element dictionary (see 14.9.3, "Alternate descriptions" and 14.9.4, "Replacement text"). NOTE

Alt is a description of the graphics objects enclosed by the Figure element, whereas ActualText gives the exact text equivalent of a graphical illustration that has the appearance of text.

Table 373 — Standard structure type Figure Structure Type

Category

Description

Figure

Grouping, Block or Inline

Encloses graphical content. The BBox attribute (see 14.8.5, "Standard structure attributes") should be present for a figure appearing in its entirety on a single page to indicate the area of the figure on the page. EXAMPLE 1

14.8.4.8.6

Some examples of content that would be marked as Figure include: •

an image

•

a drawing

•

a chart, including the text that denotes values on each axis

Formula structure type

"Table 374 — Standard structure type Formula" defines the Formula standard structure type. The standard structure type Formula shall not appear between the BT and ET operators delimiting a text object (see 9.4, "Text objects"). A Formula element may have logical substructure, including other Formula elements. For repurposing purposes it may be treated as visually static, without examining its internal contents. It should have a BBox attribute (see 14.8.5, "Standard structure attributes"). For repurposing and accessibility purposes, a Formula element should have either an Alt entry or an ActualText entry in its structure element dictionary (see 14.9.3, "Alternate descriptions" and 14.9.4, "Replacement text"). © ISO 2020 – All rights reserved

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NOTE

Alt is a description of the content enclosed by the Formula element, whereas ActualText gives the exact text equivalent of a formula has the appearance of text.

Table 374 — Standard structure type Formula Structure Type

Category

Description

Formula

Grouping, Block or Inline

Encloses a formula. The BBox attribute ("see 14.8.5, "Standard structure attributes") should be present for a formula appearing in its entirety on a single page to indicate the area of the formula on the page. EXAMPLE

14.8.4.8.7

Some examples of content that would be marked as Formula include: •

a mathematical equation or a part thereof

•

a chemical formula

•

a mathematical proof

Artifact structure type

"Table 375 — standard structure type Artifact" defines the Artifact standard structure type. Table 375 — standard structure type Artifact Structure Type

Category

Description

Artifact

Grouping, Block or Inline

(PDF 2.0) Encloses content for which semantics require a reference in the structure tree even when such content is not part of the document’s real content. The Artifact structure type may be used to enclose content that would not otherwise be tagged based on the rules of tagged PDF. A processor of tagged PDF should normally ignore any content items and structure elements that are direct or indirect descendants of an Artifact structure element. EXAMPLE

770

Some documents include pages with line numbers. Whereas for tagged PDF such line numbers would typically be considered artifacts, the Artifact structure element allows authors to ensure context is maintained by making it possible to place the line numbers in the logical structure without forcing end users to consume them as part of the logical content order.

ISO 32000-2:2020(E)

14.8.5

Standard structure attributes

14.8.5.1

General

In addition to the standard structure types, PDF defines standard structure attributes for standard structure elements in addition to those attributes in a structure element dictionary that are already defined in "Table 355 — Entries in a structure element dictionary". The example in 14.7.7, "Example of logical structure" illustrates the use of standard structure attributes. As discussed in 14.7.6, "Structure attributes" attributes shall be defined in attribute objects, which are dictionaries or streams attached to a structure element in either of two ways: The A entry in the structure element dictionary identifies an attribute object or an array of such objects. The C entry in the structure element dictionary gives the name of an attribute class or an array of such names. The class name is in turn looked up in the class map, a dictionary identified by the ClassMap entry in the structure tree root, yielding an attribute object or array of objects corresponding to the class. In addition to the standard structure attributes described in 14.8.5.2, "Standard Attribute Owners" there are several other optional entries – Lang, Alt, ActualText, and E – that are described in 14.9, "Repurposing and accessibility support" but are useful to other PDF consumers as well. They appear in the following places in a PDF file (rather than in attribute dictionaries): •

As entries in the structure element dictionary (see "Table 355 — Entries in a structure element dictionary");

•

As entries in property lists attached to marked-content sequences with the tag Span (see 14.6, "Marked content").

14.8.5.2

Standard attribute owners

Each attribute object has an owner, specified by the object’s O entry, or, if the value of O is NSO, by the object’s NS entry, which determines the interpretation of the attributes defined in the object’s dictionary. Multiple owners may define like-named attributes with different value types or interpretations. Tagged PDF defines a set of standard attribute owners as shown in "Table 376 — Standard structure attribute owners". Table 376 — Standard structure attribute owners Owner value for the attribute object’s O entry

Description

Layout

Attributes governing the layout of content

List

Attributes governing the numbering of lists

PrintField

Attributes governing Form structure elements for non-interactive form fields

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Owner value for the attribute object’s O entry

Description

Table

Attributes governing the organisation of cells in tables

Artifact

Attributes governing Artifact structure elements

XML-1.00

Additional attributes governing translation to XML, version 1.00

HTML-3.20

Additional attributes governing translation to HTML, version 3.20

HTML-4.01

Additional attributes governing translation to HTML, version 4.0

HTML-5.00

Additional attributes governing translation to HTML, version 5.0

OEB-1.00

Additional attributes governing translation to OEB (Open eBook), version 1.0

RTF-1.05

Additional attributes governing translation to Microsoft Rich Text Format, version 1.05

CSS-1

Additional attributes governing translation to a format using CSS, version 1

CSS-2

Additional attributes governing translation to a format using CSS, version 2.1

CSS-3

Additional attributes governing translation to a format using CSS, version 3

RDFa-1.10

Additional attributes governing translation to a format using RDFa version 1.1

ARIA-1.1

Additional attributes governing translation to a format using WAI-ARIA version 1.1

NOTE

(2020) The three owner values for CSS were changed in the above table to better reflect CSS numbering.

An attribute object owned by a specific export format, such as XML-1.00, shall be applied only when processing PDF content based on that format. Such format-specific attributes shall override any corresponding attributes owned by Layout, List, PrintField, Table or Artifact. There may also be additional format-specific attributes; the set of possible attributes is open-ended and is not explicitly specified or limited by tagged PDF.

14.8.5.3

Attribute values and inheritance

Some attributes are defined as inheritable. Inheritable attributes propagate down the structure tree; that is, an attribute that is specified for an element shall apply to all the descendants of the element in the structure tree unless a descendent element specifies an explicit value for the attribute. NOTE 1

The description of each of the standard attributes in this subclause specifies whether their values are inheritable.

An inheritable attribute may be specified for an element for the purpose of propagating its value to child elements, even if the attribute is not meaningful for the parent element. Non-inheritable attributes may be specified only for elements on which they would be meaningful. The following list shows the priority for determining attribute values. A PDF processor determines an attribute’s value to be the first item in the following list that applies: 772

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•

The value of the attribute specified in the element’s A entry, owned by an owner as specified by the O entry, or, if the value of the O entry is NSO, the NS entry, excluding Layout, PrintField, Table, List and Artifact, if present, and if processing based on the format indicated by the owner value

•

The value of the attribute specified in the element’s A entry, owned by Layout, PrintField, Table, List or Artifact, if present

•

The value of the attribute specified in a class map associated with the element’s C entry, if there is one

•

The resolved value of the parent structure element, if the attribute is inheritable

•

The default value for the attribute, if there is one

NOTE 2

The properties Lang, Alt, ActualText, and E do not appear in attribute dictionaries. The rules governing their application are discussed in 14.9, "Repurposing and accessibility support".

There is no semantic distinction between attributes that are specified explicitly and ones that are inherited. Logically, the structure tree has attributes fully bound to each element, even though some may be inherited from an ancestor element. This is consistent with the behaviour of properties (such as font characteristics) that are not specified by structure attributes but shall be derived from the content.

14.8.5.4

Layout attributes

14.8.5.4.1

General

Layout attributes specify parameters of the layout process used to produce the appearance described by a document’s PDF content. Attributes in this category shall be defined in attribute objects whose O (owner) entry has the value Layout or whose owner is any other owner excluding List, Table, PrintField and Artifact. NOTE 1

The intent is that these parameters can be used to repurpose the content or export it to some other document format with at least basic styling preserved.

"Table 377 — Standard layout attributes" summarizes the standard layout attributes and the structure elements to which they apply. As described in 14.8.5.3, "Attribute Values and Inheritance" an inheritable attribute may be specified for any element to propagate it to descendants, regardless of whether it is meaningful for that element. Table 377 — Standard layout attributes Structure Elements

Attributes

Inheritable

Any structure element

Placement

No

WritingMode

Yes

BackgroundColor

No

BorderColor

Yes

BorderStyle

No

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Structure Elements

Any BLSE; ILSEs with Placement other than Inline

BLSEs containing text

Figure, Form, Formula and Table elements

TH (Table header); TD (Table data)

Any ILSE; BLSEs containing ILSEs or containing direct or nested content items

Grouping elements

Vertical text

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Attributes

Inheritable

BorderThickness

Yes

Color

Yes

Padding

No

SpaceBefore

No

SpaceAfter

No

StartIndent

Yes

EndIndent

Yes

TextIndent

Yes

TextAlign

Yes

BBox

No

Width

No

Height

No

Width

No

Height

No

BlockAlign

Yes

InlineAlign

Yes

TBorderStyle

Yes

TPadding

Yes

LineHeight

Yes

BaselineShift

No

TextDecorationType

Yes, only for directly nested ILSEs

TextPosition

Yes

TextDecorationColor

Yes

TextDecorationThickness

Yes

ColumnCount

No

ColumnWidths

No

ColumnGap

No

GlyphOrientationVertical

ISO 32000-2:2020(E)

Structure Elements

Attributes

Inheritable

Ruby text

RubyAlign

Yes

RubyPosition

Yes

NOTE 2

TextPosition was corrected in the above table (2020).

14.8.5.4.2

General layout attributes

The layout attributes described in "Table 378 — Standard layout attributes common to all standard structure types" may apply to structure elements of any of the standard at the block level (BLSEs) or the inline level (ILSEs).

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Table 378 — Standard layout attributes common to all standard structure types

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Key

Type

Value

Placement

name

(Optional; not inheritable) The positioning of the element with respect to the enclosing reference area and other content. The value shall be one of the following: Block Stacked in the block-progression direction within an enclosing reference area or parent BLSE. Inline Packed in the inline-progression direction within an enclosing BLSE. Before Placed so that the before edge of the element’s allocation rectangle (see 14.8.5.4.5, "Content and Allocation Rectangles") coincides with that of the nearest enclosing reference area. The element may float, if necessary, to achieve the specified placement. The element shall be treated as a block occupying the full extent of the enclosing reference area in the inline direction. Other content shall be stacked so as to begin at the after edge of the element’s allocation rectangle. Start Placed so that the start edge of the element’s allocation rectangle (see 14.8.5.4.5, "Content and Allocation Rectangles") coincides with that of the nearest enclosing reference area. The element may float, if necessary, to achieve the specified placement. Other content that would intrude into the element’s allocation rectangle shall be laid out as a runaround. End Placed so that the end edge of the element’s allocation rectangle (see 14.8.5.4.5, "Content and Allocation Rectangles") coincides with that of the nearest enclosing reference area. The element may float, if necessary, to achieve the specified placement. Other content that would intrude into the element’s allocation rectangle shall be laid out as a runaround. When applied to an ILSE, any value except Inline shall cause the element to be treated as a BLSE instead. Default value: Block for BLSEs, Inline for ILSEs. Elements with Placement values of Before, Start, or End shall be removed from the normal stacking or packing process and allowed to float to the specified edge of the enclosing reference area or parent BLSE. Multiple such floating elements may be positioned adjacent to one another against the specified edge of the reference area or placed serially against the edge, in the order encountered. Complex cases such as floating elements that interfere with each other or do not fit on the same page may be handled differently by different PDF processors. Tagged PDF merely identifies the elements as floating and indicates their desired placement.

ISO 32000-2:2020(E)

Key

Type

Value

WritingMode

name

(Optional; inheritable) Indicates the directions of layout progression inside Block Level Structure Elements (BLSEs) (inline progression) and regarding the sequence of BLSEs (block progression). WritingMode may be used as an attribute for any structure element. The value shall be one of the following: LrTb Inline progression from left to right; block progression from top to bottom. This is the typical writing mode for Western writing systems. RlTb Inline progression from right to left; block progression from top to bottom. This is the typical writing mode for Arabic and Hebrew writing systems. TbRl Inline progression from top to bottom; block progression from right to left. This is the typical writing mode for Chinese and Japanese writing systems. TbLr Inline progression from top to bottom; block progression from left to right. This is the typical writing mode for writing systems like classical Mongolian. LrBt Inline progression from left to right; block progression from bottom to top. There is currently no known writing system to which this writing mode applies. RlBt Inline progression from right to left; block progression from bottom to top. There is currently no known writing system to which this writing mode applies. BtRl Inline progression from bottom to top; block progression from right to left. This is the typical writing mode for the Ancient Berber writing system. BtLr Inline progression from bottom to top; block progression from left to right. This is the typical writing mode for the Batak writing system. The specified layout directions shall apply to the given structure element and all of its descendants. Default value: LrTb. For elements that are represented in multiple columns, the writing mode defines the direction of column progression within the reference area: the inline direction determines the stacking direction for columns and the default flow order of text from column to column. For tables, the writing mode controls the layout of rows and columns: table rows (structure type TR) shall be stacked in the block direction, cells within a row (structure types TH and TD) in the inline direction. The inline-progression direction specified by the writing mode is subject to local override within the text being laid out, as described in Unicode Standard Annex #9, Unicode Bidirectional Algorithm, available from the Unicode Consortium.

BackgroundColor array

(Optional; not inheritable; PDF 1.5) The colour to be used to fill the background of a table cell or any element’s content rectangle (possibly adjusted by the Padding attribute). The value shall be an array of three numbers in the range 0.0 to 1.0, representing the red, green, and blue values, respectively, of an RGB colour space. If this attribute is not specified, the element shall be treated as if its background were transparent.

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Key

Type

Value

BorderColor

array

(Optional; inheritable; PDF 1.5) The colour of the border drawn on the edges of a table cell or any element’s content rectangle (possibly adjusted by the Padding attribute). The value of each edge shall be an array of three numbers in the range 0.0 to 1.0, representing the red, green, and blue values, respectively, of an RGB colour space. There are two forms: •

A single array of three numbers representing the RGB values to apply to all four edges.

•

An array of four arrays, each specifying the RGB values for one edge of the border, in the order of the before, after, start, and end edges. A value of null for any of the edges means that it shall not be drawn.

If this attribute is not specified, the border colour for this element shall be the current text fill colour in effect at the start of its associated content. BorderStyle

name or array

(Optional; not inheritable; PDF 1.5) The style of an element’s border. Specifies the stroke pattern of each edge of a table cell or any element’s content rectangle (possibly adjusted by the Padding attribute). There are two forms: •

An array of four entries, each entry specifying the style for one edge of the border in the order of the before, after, start, and end edges. A value of null for any of the edges means that it shall not be drawn.

•

A name from the list below representing the border style to apply to all four edges.

Valid values are: None No border. Forces the computed value of BorderThickness to be 0. Hidden Same as None, except in terms of border conflict resolution for table elements. Dotted The border is a series of dots. Dashed The border is a series of short line segments. Solid The border is a single line segment. Double The border is two solid lines. The sum of the two lines and the space between them equals the value of BorderThickness. Groove The border looks as though it were carved into the canvas. Ridge The border looks as though it were coming out of the canvas (the opposite of Groove). Inset The border makes the entire box look as though it were embedded in the canvas. Outset The border makes the entire box look as though it were coming out of the canvas (the opposite of Inset). Default value: None All borders shall be drawn on top of the box’s background. The colour of borders drawn for values of Groove, Ridge, Inset, and Outset shall depend on the structure element’s BorderColor attribute and the colour of the background over which the border is being drawn. NOTE Conforming HTML applications may interpret Dotted, Dashed, Double, Groove, Ridge, Inset, and Outset to be Solid.

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Key

Type

Value

BorderThickness

number or array

(Optional; inheritable; PDF 1.5) The thickness of the border drawn on the edges of a table cell or any element’s content rectangle (possibly adjusted by the Padding attribute). The value of each edge shall be a positive number in default user space units representing the border’s thickness (a value of 0 indicates that the border shall not be drawn). There are two forms: •

A number representing the border thickness for all four edges.

•

An array of four entries, each entry specifying the thickness for one edge of the border, in the order of the before, after, start, and end edges. A value of null for any of the edges means that it shall not be drawn.

Default value: 0. Padding

number or array

(Optional; not inheritable; PDF 1.5) Specifies an offset to account for the separation between the element’s content rectangle and the surrounding border (see 14.8.5.4.5, "Content and Allocation Rectangles"). A positive value enlarges the background area; a negative value trims it, possibly allowing the border to overlap the element’s text or graphic. There are two forms: •

A number representing the width of the padding for all four edges.

•

An array of four entries, each entry specifying the width of the padding for one edge, in the order of the before, after, start and end edges.

Default value: 0. Color

14.8.5.4.3

array

(Optional; inheritable; PDF 1.5) The colour to be used for drawing text and the default value for the colour of table borders and text decorations. The value shall be an array of three numbers in the range 0.0 to 1.0, representing the red, green, and blue values, respectively, of an RGB colour space. If this attribute is not specified, the border colour for this element shall be the current text fill colour in effect at the start of its associated content.

Layout Attributes for BLSEs

"Table 379 — Additional standard layout attributes specific to block-level structure elements" describes layout attributes that shall apply only to block-level structure elements (BLSEs).

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Table 379 — Additional standard layout attributes specific to block-level structure elements

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Key

Type

Value

SpaceBefore

number

(Optional; not inheritable) The amount of extra space preceding the before edge of the BLSE, measured in default user space units in the blockprogression direction. This value shall be added to any adjustments induced by the LineHeight attributes of ILSEs within the first line of the BLSE (see 14.8.5.4.4, "Layout Attributes for ILSEs"). If the preceding BLSE has a SpaceAfter attribute, the greater of the two attribute values shall be used. Default value: 0. This attribute shall be disregarded for the first BLSE placed in a given reference area.

SpaceAfter

number

(Optional; not inheritable) The amount of extra space following the after edge of the BLSE, measured in default user space units in the blockprogression direction. This value shall be added to any adjustments induced by the LineHeight attributes of ILSEs within the last line of the BLSE (see 14.8.5.4, "Layout Attributes"). If the following BLSE has a SpaceBefore attribute, the greater of the two attribute values shall be used. Default value: 0. This attribute shall be disregarded for the last BLSE placed in a given reference area.

StartIndent

number

(Optional; inheritable) The distance from the start edge of the reference area to that of the BLSE, measured in default user space units in the inlineprogression direction. This attribute shall apply only to structure elements with a Placement attribute of Block or Start (see 14.8.5.4.2, "General Layout Attributes"). The attribute shall be disregarded for elements with other Placement values. Default value: 0. A negative value for this attribute places the start edge of the BLSE outside that of the reference area. The results are implementation-dependent and may not be supported by all conforming products that process tagged PDF or by particular export formats. If a structure element with a StartIndent attribute is placed adjacent to a floating element with a Placement attribute of Start, the actual value used for the element’s starting indent shall be its own StartIndent attribute or the inline extent of the adjacent floating element, whichever is greater. This value may be further adjusted by the element’s TextIndent attribute, if any.

EndIndent

number

(Optional; inheritable) The distance from the end edge of the BLSE to that of the reference area, measured in default user space units in the inlineprogression direction. This attribute shall apply only to structure elements with a Placement attribute of Block or End (see 14.8.5.4.2, "General Layout Attributes"). The attribute shall be disregarded for elements with other Placement values. Default value: 0. A negative value for this attribute places the end edge of the BLSE outside that of the reference area. The results are implementation-dependent and may not be supported by all conforming products that process tagged PDF or by particular export formats. If a structure element with an EndIndent attribute is placed adjacent to a floating element with a Placement attribute of End, the actual value used for the element’s ending indent shall be its own EndIndent attribute or the inline extent of the adjacent floating element, whichever is greater. © ISO 2020 – All rights reserved

ISO 32000-2:2020(E)

Key

Type

Value

TextIndent

number

(Optional; inheritable; applies only to some BLSEs) The additional distance, measured in default user space units in the inline-progression direction, from the start edge of the BLSE, as specified by StartIndent, to that of the first line of text. A negative value shall indicate a hanging indent. Default value: 0. This attribute shall apply only to paragraphlike BLSEs and those of structure types LI (List item), TH (Table header), and TD (Table data), provided that they contain content other than nested BLSEs.

TextAlign

name

(Optional; inheritable; applies only to BLSEs containing text) The alignment, in the inline-progression direction, of text and other content within lines of the BLSE. Valid values are: Start Aligned with the start edge. Center Centred between the start and end edges. End Aligned with the end edge. Justify Aligned with both the start and end edges, with internal spacing within each line expanded, if necessary, to achieve such alignment. The last (or only) line shall be aligned with the start edge only. Default value: Start.

BBox

rectangle (Optional; not inheritable) An array of four numbers in default user space units that shall give the coordinates of the left, bottom, right, and top edges, respectively, of the structure element’s bounding box (the rectangle that completely encloses its visible content). The BBox attribute should be present for structure elements whose content does not lend itself to reflow or any other visual rearrangement of the content inside it. NOTE 1 Examples of types of structure elements that do not lend themselves to reflow include Figure and Formula structure elements. NOTE 2 The semantics of the visual presentation of charts, illustrations consisting of more than one graphics object, or formulas can suffer if the objects inside them are rearranged, as is typical for content reflow.

A structure element with a BBox attribute may contain other structure elements inside it. NOTE 3 A formula, for example, can lose its meaning if the parts in the formula are visually rearranged. At the same time, the parts inside the formula could be individually tagged, for example with inline level structure elements. EXAMPLE

Formulas, graphic art, vector drawings, images are types of structure elements for which a BBox attribute is appropriate.

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Key

Type

Value

Width

number or name

(Optional; not inheritable; illustrations, tables, and table cells only; should be used for table cells; sometimes required for Figure, Form or Formula elements with Placement attribute) The width of the element’s content rectangle (see 14.8.5.4.5, "Content and Allocation Rectangles"), measured in default user space units in the inline-progression direction. This attribute shall apply only to elements of structure type Figure, Formula, Table, TH (Table header), or TD (Table data). The name Auto in place of a numeric value shall indicate that no specific width constraint is to be imposed; the element’s width is determined by the intrinsic width of its content. Default value: Auto.

Height

number or name

(Optional; not inheritable; illustrations, tables, table headers, and table cells only; sometimes required for Figure, Form or Formula elements with Placement attribute) The height of the element’s content rectangle (see 14.8.5.4.5, "Content and Allocation Rectangles"), measured in default user space units in the block-progression direction. This attribute shall apply only to elements of structure type Figure, Formula, Table, TH (Table header), or TD (Table data). The name Auto in place of a numeric value shall indicate that no specific height constraint is to be imposed; the element’s height is determined by the intrinsic height of its content. Default value: Auto.

BlockAlign

name

(Optional; inheritable; table cells only) The alignment, in the blockprogression direction, of content within the table cell. Valid values are: Before Before edge of the first child’s allocation rectangle aligned with that of the table cell’s content rectangle. Middle Children centred within the table cell. The distance between the before edge of the first child’s allocation rectangle and that of the table cell’s content rectangle shall be the same as the distance between the after edge of the last child’s allocation rectangle and that of the table cell’s content rectangle. After After edge of the last child’s allocation rectangle aligned with that of the table cell’s content rectangle. Justify Children aligned with both the before and after edges of the table cell’s content rectangle. The first child shall be placed as described for Before and the last child as described for After, with equal spacing between the children. If there is only one child, it shall be aligned with the before edge only, as for Before. This attribute shall apply only to elements of structure type TH (Table header) or TD (Table data) and shall control the placement of all BLSEs that are children of the given element. The table cell’s content rectangle (see 14.8.5.4.5, "Content and Allocation Rectangles") shall become the reference area for all of its descendants. Default value: Before.

ISO 32000-2:2020(E)

Key

Type

Value

InlineAlign

name

(Optional; inheritable; table cells only) The alignment, in the inlineprogression direction, of content within the table cell. Valid values are: Start Start edge of each child’s allocation rectangle aligned with that of the table cell’s content rectangle. Center Each child centred within the table cell. The distance between the start edges of the child’s allocation rectangle and the table cell’s content rectangle shall be the same as the distance between their end edges. End End edge of each child’s allocation rectangle aligned with that of the table cell’s content rectangle. This attribute shall apply only to elements of structure type TH (Table header) or TD (Table data) and controls the placement of all ILSEs that are children of the given element. The table cell’s content rectangle (see 14.8.5.4.5, "Content and Allocation Rectangles") shall become the reference area for all of its descendants. Default value: Start.

TBorderStyle name or array

(Optional; inheritable; PDF 1.5) The style of the border drawn on each edge of a table cell. Allowed values shall be the same as those specified for BorderStyle (see "Table 379 — Additional standard layout attributes specific to block-level structure elements"). If both TBorderStyle and BorderStyle apply to a given table cell, BorderStyle shall supersede TBorderStyle. Default value: None.

TPadding

(Optional; inheritable; PDF 1.5) Specifies an offset to account for the separation between the table cell’s content rectangle and the surrounding border (see 14.8.5.4.5, "Content and Allocation Rectangles"). If both TPadding and Padding apply to a given table cell, Padding shall supersede TPadding. A positive value shall enlarge the background area; a negative value shall trim it, and the border may overlap the element’s text or graphic. The value shall be either a single number representing the width of the padding, in default user space units, that applies to all four edges of the table cell, or a 4-entry array representing the padding width for the before edge, after edge, start edge, and end edge, respectively, of the content rectangle. Default value: 0.

14.8.5.4.4

integer or array

Layout Attributes for ILSEs

The attributes described in "Table 380 — Standard layout attributes specific to inline-level structure elements" apply to inline-level structure elements (ILSEs). They may also be specified for a block-level element (BLSE) and may apply to any content items that are its immediate children.

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Table 380 — Standard layout attributes specific to inline-level structure elements Key

Type

Value

BaselineShift

number

(Optional; not inheritable) The distance, in default user space units, by which the element’s baseline shall be shifted relative to that of its parent element. The shift direction shall be the opposite of the block-progression direction specified by the prevailing WritingMode attribute (see "General Layout Attributes" in 14.8.5.4, "Layout Attributes"). Thus, positive values shall shift the baseline toward the before edge and negative values toward the after edge of the reference area (upward and downward, respectively, in Western writing systems). Default value: 0. The shifted element may be a superscript, a subscript, or an inline graphic. The shift shall apply to the element, its content, and all of its descendants. Any further baseline shift applied to a child of this element shall be measured relative to the shifted baseline of this (parent) element.

LineHeight

number (Optional; inheritable) The element’s preferred height, measured in or name default user space units in the block-progression direction. The height of a line of text is determined by the largest LineHeight value for any complete or partial ILSE that it contains. The name Normal or Auto in place of a numeric value shall indicate that no specific height constraint is to be imposed. The element’s height shall be set to a reasonable value based on the content’s font size: Normal Adjust the line height to include any non-zero value specified for BaselineShift. Auto Adjustment for the value of BaselineShift shall not be made. Default value: Normal. The meaning of the term "reasonable value" is left to the PDF processor to determine. It should be approximately 1.2 times the font size, but this value may vary depending on the export format. This attribute applies to all ILSEs (including implicit ones) that are children of this element or of its nested ILSEs, if any. It shall not apply to nested BLSEs. When translating to a specific export format, the values Normal and Auto, if specified, shall be used directly if they are available in the target format. NOTE 1 In the absence of a numeric value for LineHeight or an explicit value for the font size, a reasonable method of calculating the line height from the information in a tagged PDF file is to find the difference between the associated font’s Ascent and Descent values (see 9.8, "Font descriptors"), map it from glyph space to default user space (see 9.4.4, "Text space details"), and use the maximum resulting value for any character in the line.

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Key

Type

Value

TextPosition

name

(Optional; inheritable; PDF 2.0) The position of the element relative the immediately surrounding content. Valid values are: Sup Position is elevated, like for superscript. Sub Position is lowered, like for subscript Normal Position is neither elevated nor lowered. Default value: Normal TextPosition does not imply any specific semantic. NOTE 2 As a consequence, it cannot be determined whether text with a TextPosition attribute of Sup is a footnote number, an exponent, an index or some other use of the text. For mathematical expressions, MathML structure elements provide a richer semantic for superscripted or subscripted content.

TextDecorationColor

array

(Optional; inheritable; PDF 1.5) The colour to be used for drawing text decorations. The value shall be an array of three numbers in the range 0.0 to 1.0, representing the red, green, and blue values, respectively, of an RGB colour space. If this attribute is not specified, the text decoration colour for this element shall be the current fill colour in effect at the start of its associated content.

TextDecorationThickness number

(Optional; inheritable; PDF 1.5) The thickness of each line drawn as part of the text decoration. The value shall be a non-negative number in default user space units representing the thickness (0 is interpreted as the thinnest possible line). If this attribute is not specified, it shall be derived from the current stroke thickness in effect at the start of the element’s associated content, transformed into default user space units.

TextDecorationType

(Optional; inheritable only for directly nested ILSEs) The text decoration, if any, to be applied to the element’s text. Valid values are:

name

None

No text decoration

Underline

A line below the text

Overline

A line above the text

LineThrough A line through the middle of the text Default value: None. This attribute shall apply to all text content items that are children of this element or of its nested ILSEs, if any. The attribute shall not apply to nested BLSEs or to content items other than text. The colour, position, and thickness of the decoration shall be uniform across all children, regardless of changes in colour, font size, or other variations in the content’s text characteristics.

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Key

Type

Value

RubyAlign

name

(Optional; inheritable; PDF 1.5) The justification of the lines within a ruby assembly. Valid values are: Start

The content shall be aligned on the start edge in the inline-progression direction.

Center

The content shall be centred in the inline-progression direction.

End

The content shall be aligned on the end edge in the inline-progression direction.

Justify

The content shall be expanded to fill the available width in the inline-progression direction.

Distribute

The content shall be expanded to fill the available width in the inline-progression direction. However, space shall also be inserted at the start edge and end edge of the text. The spacing shall be distributed using a 1:2:1 (start:infix:end) ratio. It shall be changed to a 0:1:1 ratio if the ruby appears at the start of a text line or to a 1:1:0 ratio if the ruby appears at the end of the text line.

Default value: Distribute. This attribute may be specified on the RB and RT elements. When a ruby is formatted, the attribute shall be applied to the shorter line of these two elements. For example, if the RT element has a shorter width than the RB element, the RT element shall be aligned as specified in its RubyAlign attribute. RubyPosition

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name

(Optional; inheritable; PDF 1.5) The placement of the RT structure element relative to the RB element in a ruby assembly. Valid values are: Before The RT content shall be aligned along the before edge of the element. After The RT content shall be aligned along the after edge of the element. WarichuThe RT and associated RP elements shall be formatted as a warichu, following the RB element. Inline The RT and associated RP elements shall be formatted as a parenthesis comment, following the RB element. Default value: Before.

ISO 32000-2:2020(E)

Key

Type

Value

GlyphOrientationVertical number (Optional; inheritable; PDF 1.5) Specifies the orientation of glyphs or name when the inline-progression direction is top to bottom or bottom to top. Valid values are: angle A number representing the clockwise rotation in degrees of the top of the glyphs relative to the top of the reference area. Shall be a multiple of 90 degrees between -180 and +360. Auto Specifies a default orientation for text, depending on whether it is fullwidth (as wide as it is high). Fullwidth Latin and fullwidth ideographic text (excluding ideographic punctuation) shall be set with an angle of 0. Ideographic punctuation and other ideographic characters having alternate horizontal and vertical forms shall use the vertical form of the glyph. Non-fullwidth text shall be set with an angle of 90. Default value: Auto. NOTE 3 This attribute is used most commonly to differentiate between the preferred orientation of alphabetic (non-ideographic) text in vertically written Japanese documents (Auto or 90) and the orientation of the ideographic characters and/or alphabetic (nonideographic) text in western signage and advertising (90).

This attribute shall affect both the alignment and width of the glyphs. If a glyph is perpendicular to the vertical baseline, its horizontal alignment point shall be aligned with the alignment baseline for the script to which the glyph belongs. The width of the glyph area shall be determined from the horizontal width font characteristic for the glyph.

14.8.5.4.5

Content and allocation rectangles

As defined in 14.8.3, "Basic Layout Model" an element’s content rectangle is an enclosing rectangle derived from the shape of the element’s content, which shall define the bounds used for the layout of any included child elements. The allocation rectangle includes any additional borders or spacing surrounding the element, affecting how it shall be positioned with respect to adjacent elements and the enclosing content rectangle or reference area. The exact definition of the content rectangle shall depend on the element’s structure type: •

For a table cell (structure type TH or TD), the content rectangle is determined from the bounding box of all graphics objects in the cell’s content, taking into account any explicit bounding boxes (such as the BBox entry in a form XObject). This implied size may be explicitly overridden by the cell’s Width and Height attributes. The cell’s height shall be adjusted to equal the maximum height of any cell in its row; its width shall be adjusted to the maximum width of any cell in its column.

•

For any other BLSE other than TH and TD, the height of the content rectangle shall be the sum of the heights of all BLSEs it contains, plus any additional spacing adjustments between these elements.

For an ILSE that contains text, the height of the content rectangle shall be set by the LineHeight attribute. The width shall be determined by summing the widths of the contained characters, adjusted for any indents, letter spacing, word spacing, or line-end conditions. © ISO 2020 – All rights reserved 787 •

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•

For an ILSE that contains an illustration or table, the content rectangle shall be determined from the bounding box of all graphics objects in the content, and shall take into account any explicit bounding boxes (such as the BBox entry in a form XObject). This implied size may be explicitly overridden by the element’s Width and Height attributes.

•

For an ILSE that contains a mixture of elements, the height of the content rectangle shall be determined by aligning the child objects relative to one another based on their text baseline (for text ILSEs) or end edge (for non-text ILSEs), along with any applicable BaselineShift attribute (for all ILSEs), and finding the extreme top and bottom for all elements.

NOTE

PDF processors can apply this process to all elements within the block or apply it on a line-byline basis.

The allocation rectangle shall be derived from the content rectangle in a way that also depends on the structure type: •

For a BLSE, the allocation rectangle shall be equal to the content rectangle with its before and after edges adjusted by the element’s SpaceBefore and SpaceAfter attributes, if any, but with no changes to the start and end edges.

•

For an ILSE, the allocation rectangle is the same as the content rectangle.

14.8.5.4.6

Figure, Form and Formula attributes

Particular uses of Figure, Form or Formula elements shall have additional restrictions: •

When a Figure, Form or Formula element has a Placement attribute of Block, it shall have a Height attribute with an explicitly specified numerical value (not Auto). This value shall be the sole source of information about the element’s extent in the block-progression direction.

•

When a Figure, Form or Formula element has a Placement attribute of Inline, it shall have a Width attribute with an explicitly specified numerical value (not Auto). This value shall be the sole source of information about the element’s extent in the inline-progression direction.

•

When a Figure, Form or Formula element has a Placement attribute of Inline, Start, or End, the value of its BaselineShift attribute shall be used to determine the position of its after edge relative to the text baseline; BaselineShift shall be ignored for all other values of Placement. (A Figure, Form or Formula element with a Placement value of Start may be used to create a dropped capital; one with a Placement value of Inline may be used to create a raised capital.)

14.8.5.4.7

Column attributes

The attributes described in "Table 381 — Standard layout attributes specific to standard column attributes" shall be present for the grouping elements (see 14.8.4.4, "Grouping level structure types") if the content in the grouping element is divided into columns. Table 381 — Standard layout attributes specific to standard column attributes

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Key

Type

Value

ColumnCount

integer

(Optional; not inheritable; PDF 1.6) The number of columns in the content of the grouping element. Default value: 1.

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Key

Type

Value

ColumnGap

number or array

(Optional; not inheritable; PDF 1.6) The desired space between adjacent columns, measured in default user space units in the inline-progression direction. If the value is a number, it specifies the space between all columns. If the value is an array, it should contain numbers, the first element specifying the space between the first and second columns, the second specifying the space between the second and third columns, and so on. If there are fewer than ColumnCount - 1 numbers, the last element shall specify all remaining spaces; if there are more than ColumnCount

1 numbers, the excess array elements shall be ignored.

ColumnWidths

number or array

(Optional; not inheritable; PDF 1.6) The desired width of the columns, measured in default user space units in the inlineprogression direction. If the value is a number, it specifies the width of all columns. If the value is an array, it shall contain numbers, representing the width of each column, in order. If there are fewer than ColumnCount numbers, the last element shall specify all remaining widths; if there are more than ColumnCount numbers, the excess array elements shall be ignored.

14.8.5.5

List attributes

If present, the List attributes described in "Table 382 — Standard list attributes" shall appear in an L (List) element. These attributes control the interpretation of the Lbl (Label) elements (see “Table 368 — General inline level structure types” within the list’s LI (List Item) elements (see 14.8.4.8.2, "Standard structure types for Lists"). These attributes may only be defined in attribute objects whose O (owner) entry has the value List or whose owner is any other owner excluding Layout, Table, PrintField and Artifact. The ContinuedList and the ContinuedFrom attributes described in "Table 382 — Standard list attributes" control the interpretation of the L element as it relates to other L elements that are not its immediate parent.

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Table 382 — Standard list attributes Key

Type

Value

ListNumbering

name

(Optional; inheritable) The numbering system used to generate the content of the Lbl (Label) elements in a numbered list, or the type of symbol in the content of the Lbl (Label) elements used to identify list items in an unnumbered list (see "Table 368 — General inline level structure types"). The value of the ListNumbering shall be one of the following, and shall be applied as described here. None No numbering system; Lbl elements (if present) contain arbitrary text not subject to any numbering scheme Unordered (PDF 2.0) Unordered list with unspecified bullets Description (PDF 2.0) A list of terms for corresponding definitions NOTE

The Description value was added in this document (2020).

Disc Solid circular bullet Circle Open circular bullet Square Solid square Ordered (PDF 2.0) Ordered lists with unspecified numbering Decimal Decimal Arabic numerals (1–9, 10–99, … ) UpperRoman Uppercase Roman numerals (I, II, III, IV, … ) LowerRoman Lowercase Roman numerals (i, ii, iii, iv, … ) UpperAlpha Uppercase letters (A, B, C, .. ) LowerAlpha Lowercase letters (a, b, c, … ) Default value: None. A list is an unordered list unless the ListNumbering attribute is present with one of the following values: Ordered, Decimal, UpperRoman, LowerRoman, UpperAlpha or LowerAlpha, in which case the list is an ordered list. The alphabet used for UpperAlpha and LowerAlpha is determined by the prevailing Lang entry (see 14.9.2, "Natural language specification"). ContinuedList

boolean

ContinuedFrom ID (byte string)

NOTE

14.8.5.6

(Optional; PDF 2.0) A flag specifying whether the list is a continuation of a previous list in the structure tree (true), or not (false). Default value: false. If the ContinuedFrom attribute is not present, the continuation is from the preceding list at the same level in the structure hierarchy. (Optional; PDF 2.0) The ID (see “Table 355 — Entries in a structure element dictionary") of the list for which this list is a continuation.

The ListNumbering attribute allows a content extraction tool to autonumber a list. However, the list’s Lbl structure elements can contain the resulting numbers explicitly, so that the document can be reused without autonumbering.

PrintField attributes

The attributes described in the next table define the accessibility mechanism for non-interactive PDF forms (see 12.7.9, "Non-interactive forms"). Such forms may have originally contained interactive fields such as text fields and radio buttons but were then converted into non-interactive PDF files, or they may have been designed to be printed out and filled in manually. Since the form’s fields cannot be determined from interactive elements, form field roles and values in a non-interactive form field are defined using a PrintField attribute on respective Form elements (see "Table 383 — PrintField 790

ISO 32000-2:2020(E)

attributes") enclosing each set of content representing a non-interactive form field. This attribute may only be defined in attribute objects whose O (owner) entry has the value PrintField or whose owner is any other owner excluding Layout, List, Table and Artifact. Table 383 — PrintField attributes Key

Type

Value

Role

name

(Optional; not inheritable; PDF 1.7) The type of form field represented. The value of Role shall be one of the following: rb Radio button cb

Check box

pb

Push button

tv Text-value field lb Listbox field The tv role shall be used for non-interactive fields with textual values. The text that is the value of the field shall be the content of the Form structure element (see "Table 368 — General inline level structure types"). NOTE 1 Examples include text edit fields, numeric fields, password fields, digital signature fields and combo box fields.

Semantic groupings of non-interactive form fields and associated content (for example, a set of radio button fields associated with a label) should be enclosed within a Part structure element. Default value: None specified. Checked, checked

name

(Optional; not inheritable; PDF 1.7; lower case form is deprecated in PDF 2.0) The state of a radio button or check box field. The value shall be one of: on, off, or neutral. NOTE 2 In earlier versions of PDF the case (capitalization) used for this key did not conform to the same conventions used elsewhere in this standard.

Default value: off. Desc

text string

(Optional; not inheritable; PDF 1.7) The alternate name of the field. NOTE 3 Similar to the value supplied in the TU entry of the field dictionary for interactive fields (see "Table 226 — Entries common to all field dictionaries").

14.8.5.7

Table attributes

The table attributes, as described in "Table 384 — Standard table attributes", may only be defined in attribute objects whose O (owner) entry has the value Table or whose owner is any other owner excluding Layout, List, PrintField and Artifact.

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Table 384 — Standard table attributes Key

Type

Value

RowSpan

integer

(Optional; not inheritable) The number of rows in the enclosing table that shall be spanned by the cell. The cell shall expand by adding rows in the block-progression direction specified by the table’s WritingMode attribute. Default value: 1. This entry shall only have an effect for structure elements of type of TH or TD.

ColSpan

integer

(Optional; not inheritable) The number of columns in the enclosing table that shall be spanned by the cell. The cell shall expand by adding columns in the inline-progression direction specified by the table’s WritingMode attribute. Default value: 1. This entry shall only have an effect for structure elements of type of TH or TD.

Headers

array

(Optional; not inheritable) An array of byte strings, where each string shall be the element identifier (see the ID entry in "Table 355 — Entries in a structure element dictionary") for a TH structure element that shall be used as a header associated with this cell. This entry shall only have an effect for structure elements of type of TH or TD. The order in which the entries in the Headers array are listed shall be row IDs followed by column IDs. The row and column IDs shall be ordered from most specific to most general. If the scope for any cells with an ID listed in the Headers attribute of a cell cannot be determined by the default algorithm defined in 14.8.4.8.3, "Table structure types", those header cells shall specify a Scope so that the header can be determined to be either a row header, a column header or both. This attribute may apply to header cells (TH) as well as data cells (TD). Therefore, the headers associated with any cell shall be those in its Headers array plus those in the Headers array of any TH cells in that array, and so on recursively.

Scope

name

(Optional; not inheritable; PDF 1.5) A name whose value shall be one of the following: Row, Column, or Both. This entry shall only have an effect for structure elements of type of TH. If a Scope is not specified for a TH structure element, then the assumed value for the Scope shall be determined as follows, taking into account the current value for WritingMode: •

if it is in the first row and column, the Scope is assumed to be Both;

•

otherwise, if it is in the first row, the Scope is assumed to be Column.

•

otherwise, if it is in the first column, the Scope is assumed to be Row.

•

otherwise, the Scope is assumed to be Both.

These assumptions are used by the algorithm following "Table 371 — Table standard structure types" for determining which headers are associated with a cell.

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Key

Type

Value

Summary

text string

(Optional; not inheritable; PDF 1.7) A summary of the table’s purpose and structure. This entry shall only be used within Table structure elements (see Table 371 — Table standard structure types”. NOTE

Short

text string

For use in non-visual rendering such as speech or braille. The Summary key was restored in this document (2020).

(Optional; not inheritable; PDF 2.0) Contains a short form of the content of a TH structure element’s content. This entry shall only have an effect for structure elements of type of TH. EXAMPLE

14.8.5.8

When accessed by means of a screen reader, for each table cell the applicable header cells are read to the user in order to allow that user to understand the content of the table cell. It can become cumbersome for a user to repeatedly have to listen to the full contents of a TH structure element. An option to have the short form of the content of the TH structure element read out aloud is sometimes preferred.

Artifact attributes

The artifact attributes described in "Table 385 — Standard artifact attributes" may only be defined in attribute objects whose O (owner) entry has the value Artifact or whose owner is any other owner excluding Layout, List, PrintField and Table. Table 385 — Standard artifact attributes Key

Type

Value

Type

Name

(Optional) The type of artifact that this attribute describes; if present, shall be one of the names Pagination, Layout, Page or Inline (PDF 2.0). •

Pagination artifacts are ancillary page features such as running heads or folios (page numbers)

•

Layout artifacts are purely cosmetic typographical or design elements such as footnote rules or decorative ornaments

•

Page artifacts are production aids extraneous to the document itself, such as cut marks and print control patches

•

Inline artifacts enclose artifact content that has context in the document’s logical structure, typically, artifacts of subtype LineNum or Redaction

NOTE

BBox

rectangle

Inline artifacts can be used to provide context in the logical structure to any artifact. This is similar to an inline structure element.

(Optional) An array of four numbers in default user space units giving the coordinates of the left, bottom, right, and top edges, respectively, of the artifact’s bounding box (the rectangle that completely encloses its visible extent).

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Key

Type

Value

Subtype

Name

(Optional; PDF 1.7) The subtype of the artifact. This entry should appear only when the Type entry has a value of Pagination or Inline. Valid values are Header, Footer, Watermark, PageNum (PDF 2.0), Bates (PDF 2.0), LineNum (PDF 2.0) and Redaction (PDF 2.0). Additional values may be specified for this entry, provided they comply with the naming conventions described in Annex E, "Extending PDF".

14.8.6

Standard structure namespaces

14.8.6.1

Namespaces for standard structure types and attributes

The namespace mechanism defined as part of logical structure (see 14.7.4, "Namespaces") in PDF 2.0 defines a more robust means of interchanging tagsets than was previously possible. The standard structure types and attributes defined within the previous clauses (see 14.8.4, "Standard structure types" and 14.8.5, "Standard structure attributes") effectively define a schema for a tagset in PDF. This schema is called the standard structure namespace for PDF 2.0, which shall be defined by the namespace name: "http://iso.org/pdf2/ssn" ISO 32000-1:2008, Clause 14.8, "Tagged PDF" describes the schema for the standard structure types and attributes that were defined prior to PDF 2.0 and the rules for processing documents tagged using them. This schema shall be known as the standard structure namespace for PDF 1.7, and shall be defined by the namespace name: "http://iso.org/pdf/ssn" To facilitate conversion of documents created against versions of the PDF standard earlier than PDF 2.0, the default standard structure namespace shall be "http://iso.org/pdf/ssn". When a namespace is not explicitly specified for a given structure element or attribute, it shall be assumed to be within this default standard structure namespace. NOTE

Annex M, "Differences between the standard structure namespaces" lists the standard structure types defined in the default (PDF 1.7) namespace.

The term standard structure namespaces refers to either of the two namespaces defined above.

14.8.6.2

Role maps and namespaces

Role maps prior to the introduction of namespaces identify a given structure element and map it to another structure element. This can be applied transitively to allow a structure element to be mapped through multiple steps to a final structure element. Tagged PDF requires that all structure elements be role mapped to a standard structure type except where explicitly stated in this subclause. The introduction of namespaces adds complexity here, since there may be multiple structure element dictionaries specifying the same structure type name (see "Table 355 — Entries in a structure element dictionary"), but from a different namespace, which require different mappings. To enable support for this, a namespace dictionary (see 14.7.4.2, "Namespace dictionary") can define a role map specific to it. 794

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When processing a structure element dictionary within a tagged PDF document, if the structure element does not explicitly identify its namespace using an NS entry, it should use the RoleMap entry in the Structure Tree Root dictionary (see "Table 354 — Entries in the structure tree root") to determine its role mapping, if any. If the structure element is in an explicit namespace, then that namespace shall be identified in the structure tree root dictionary’s Namespaces array entry and the RoleMapNS entry within that namespace dictionary shall provide the role mapping, if any. In a tagged PDF, all structure elements shall be in at least one of the standard structure namespaces or in a namespace identified in 14.8.6.3, “Other namespaces”. An element shall be considered to be in one of these namespaces if: •

they directly identify one of these namespaces through their NS entry;

•

they are in the default standard structure namespace;

•

they are role mapped into the namespace, either directly or transitively.

NOTE 1

This provision facilitates interoperability by allowing a structure element to be in multiple namespaces, including the standard structure namespace for PDF 1.7 and the standard structure namespace for PDF 2.0.

NOTE 2

The above paragraph and bullets were rewritten in this document (2020).

14.8.6.3

Other namespaces

The standard structure types (see 14.8.4, "Standard structure types") address many of the structural elements that commonly exist within general documents. However, the ILSEs defined do not provide sufficient structure to support domain specific languages. An example of such a language is mathematics, which is common to many classes of documents. This subclause identifies any domain specific languages that are common within broad ranges of documents types. Namespaces identified in this subclause do not require a RoleMapNS entry in their respective namespace dictionary. MathML 3.0 defines a domain specific schema for representing mathematics. The namespace name (see 14.7.4.2, "Namespace dictionary"), as would be identified by the NS entry in a namespace dictionary, shall have the value: “http://www.w3.org/1998/Math/MathML” NOTE 1

MathML is the only domain-specific namespace defined in PDF 2.0.

When including mathematics structured as MathML 3.0, the math structure element type as defined in MathML 3.0 shall be used, and shall have its namespace explicitly defined (see 14.7.4.2, "Namespace dictionary"). NOTE 2

The math structure element type is all lowercase to match the MathML 3.0 specification.

Any other namespaces can be specified within a PDF document, but shall meet the requirements of role mapping described in 14.8.6.2, "Role maps and namespaces".

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