Rather than relying on the tab URL containing '.pdf',
check whether the current tab is displaying Chrome's native
PDF viewer and open Hypothesis' PDF.js based viewer in that
case since we the sidebar cannot be injected into the native viewer.

We detect the Chrome PDF viewer by checking for an
<embed ... type="application/pdf"> tag which appears at the top
of the body.

If the user is using a PDF.js or other HTML-based viewer,
then Hypothesis can work with that directly.

An alternative approach would be to detect the mime type of the
loaded document but that requires intercepting all page
load requests in order to read the Content-Type header, which
is not available after the page has loaded OR firing a network
request.

See also this upstream issue: https://code.google.com/p/chromium/issues/detail?id=242575

h/browser/chrome/lib/sidebar-injector.js

@@ -6,6 +6,27 @@ var messageTypes = require('./message-types');
var settings = require('./settings');
var util = require('./util');

var CONTENT_TYPE_PDF = 'PDF';

// a function which is executed as a content script
// to determine the type of content being displayed in a tab
function detectContentType() {
  // check if this is the Chrome PDF viewer
  if (document.querySelector('embed[type="application/pdf"]')) {
    return {
      type: 'PDF',
    };
  } else {
    return {
      type: 'HTML',
    };
  }
}

function toIIFEString(fn) {
  return '(' + fn.toString() + ')()';
}

/* The SidebarInjector is used to deploy and remove the Hypothesis sidebar
 * from tabs. It also deals with loading PDF documents into the PDF.js viewer
 * when applicable.
@@ -75,8 +96,16 @@ function SidebarInjector(chromeTabs, dependencies) {
    return PDF_VIEWER_URL + '?file=' + encodeURIComponent(url);
  }

  function isPDFURL(url) {
    return url.toLowerCase().indexOf('.pdf') > 0;
  function detectTabContentType(tab) {
    if (tab.url.toLowerCase().indexOf('.pdf') > 0) {
      return Promise.resolve(CONTENT_TYPE_PDF);
    } else {
      return executeScriptFn(tab.id, {
        code: toIIFEString(detectContentType)
      }).then(function (frameResults) {
        return frameResults[0].type;
      });
    }
  }

  function isPDFViewerURL(url) {
@@ -95,15 +124,23 @@ function SidebarInjector(chromeTabs, dependencies) {
  }

  function injectIntoLocalDocument(tab) {
    if (isPDFURL(tab.url)) {
      return injectIntoLocalPDF(tab);
    } else {
      return Promise.reject(new errors.LocalFileError('Local non-PDF files are not supported'));
    }
    return detectTabContentType(tab).then(function (type) {
      if (type === CONTENT_TYPE_PDF) {
        return injectIntoLocalPDF(tab);
      } else {
        return Promise.reject(new errors.LocalFileError('Local non-PDF files are not supported'));
      }
    });
  }

  function injectIntoRemoteDocument(tab) {
    return isPDFURL(tab.url) ? injectIntoPDF(tab) : injectIntoHTML(tab);
    return detectTabContentType(tab).then(function (type) {
      if (type === CONTENT_TYPE_PDF) {
        return injectIntoPDF(tab);
      } else {
        return injectIntoHTML(tab);
      }
    });
  }

  function injectIntoPDF(tab) {