PDF files:
[1.pdf](https://github.com/mozilla/pdf.js/files/2955999/1.pdf)
[2.pdf](https://github.com/mozilla/pdf.js/files/2956000/2.pdf)
[3.pdf](https://github.com/mozilla/pdf.js/files/2956002/3.pdf)

Configuration:
- Web browser and its version: Chrome, Firefox
- Operating system and its version: Ubuntu, macOS
- PDF.js version: 2.2.42, but the same was with 2.1.x, and with https://mozilla.github.io/pdf.js/web/viewer.html
- Is a browser extension: no

Steps to reproduce the problem:
1. Select text
2. Copy text

What went wrong?

Getting this:
`OpenSansisahumanistsansseriftypefacedesignedbySteveMatteson.OpenSanswasdesignedwithanuprightstress,openformsandaneu-tral,yetfriendlyappearance.Itwasoptimizedforprint,web,andmobileinterfaces,andhasexcellentlegibilitycharacteristicsinitsletterforms(seefigureonthefollowingpage).ThisfontisavailablefromtheGoogleFontDirectory[]asTrueTypefileslicensedundertheApacheLicenseversion..`

Instead of this:
`Open Sans is a humanist sans serif typeface designed by Steve Matteson.
Open Sans was designed with an upright stress, open forms and a neutral,
yet friendly appearance. It was optimized for print, web, and mobile
interfaces, and has excellent legibility characteristics in its letterforms (see
figure 1 on the following page). This font is available from the Google Font
Directory [1] as TrueType files licensed under the Apache License version 2.0.`

This is quite significant problem, because I would say 1/20 of scientific papers encounter this problem.

}
      f (!frtWdth) {
        frtWdth = glyphWdth;
        cntnue;
      }
      f (frtWdth !== glyphWdth) {
        Mnpace = fale;
        break;
      }
    }
    f (Mnpace) {
      prperte.flag |= FntFlag.FxedPtch;
    }
    prperte.defaultWdth = defaultWdth;
    prperte.wdth = glyphWdth;
    prperte.defaultVMetrc = defaultVMetrc;
    prperte.vmetrc = glyphVMetrc;
  }
  SerfFnt(baeFntName) {
     Smulatng decrptr flag attrbute
    cnt fntNameWStyle = baeFntName.plt("-")[0];
    return (
      fntNameWStyle n getSerfFnt() ||
      fntNameWStyle.earch(/erf/g) !== -1
    );
  }
  getBaeFntMetrc(name) {
    let defaultWdth = 0;
    let wdth = Object.create(null);
    let mnpace = fale;