Attach (recommended) or Link to PDF file here:

Configuration:
- Web browser and its version: Chrome 63.0.3239.132 
- Operating system and its version: Windows 10
- PDF.js version: using your official demo project
- Is a browser extension: No

Steps to reproduce the problem:
1.  Just load attached pdf
2.

What is the expected behavior? (add screenshot)
[E-WG18_105147.pdf](https://github.com/mozilla/pdf.js/files/1634833/E-WG18_105147.pdf)

What went wrong? (add screenshot)
![image](https://user-images.githubusercontent.com/29802177/34986271-3ca71a5c-fab7-11e7-95e8-1739dda3cd04.png)

Link to a viewer (if hosted on a site other than mozilla.github.io/pdf.js or as Firefox/Chrome extension):

if (properties.isInternalFont) {
    return;
  }
  if (builtInEncoding === properties.defaultEncoding) {
    return; 
  }
  if (properties.toUnicode instanceof IdentityToUnicodeMap) {
    return;
  }
  const toUnicode = [],
    glyphsUnicodeMap = getGlyphsUnicode();
  for (const charCode in builtInEncoding) {
    if (properties.hasIncludedToUnicodeMap) {
      if (properties.toUnicode.has(charCode)) {
        continue; 
      }
    } else {
      if (
        properties.hasEncoding &&
        properties.differences[charCode] !== undefined
      ) {
        continue; 
      }
    }
    const glyphName = builtInEncoding[charCode];
    const unicode = getUnicodeForGlyph(glyphName, glyphsUnicodeMap);
    if (unicode !== -1) {