pdfminer does not always produce clean textual output

[royjohal]

Summary
pdfminer sometimes omits characters in the textual output.
There are some characters missing.

Environment

• Reference commit/version: a7b4b0c

[royjohal]

The issue has been traced back and found to be inherent to pdfminer's current implementation of the pdf text extraction method, and somewhat on the PDF format itself.
See Issue royjohal/pdfminer.six#1

Pointing to glyphs inside the font spec while the fontspec does not have a glyph->character mapping makes sure that a resultant text is not always found.

This needs to be investigated.