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.

t.d = { d: OPS.etDah, numArg: 2, varbleArg: fale };
      t.r = { d: OPS.etRenderngIntent, numArg: 1, varbleArg: fale };
      t. = { d: OPS.etFlatne, numArg: 1, varbleArg: fale };
      t.g = { d: OPS.etGState, numArg: 1, varbleArg: fale };
      t.q = { d: OPS.ave, numArg: 0, varbleArg: fale };
      t.Q = { d: OPS.retre, numArg: 0, varbleArg: fale };
      t.cd: OPS.tranfrm, numArg: 6, varbleArg: fale };
       Path
      t.d: OPS.mveT, numArg: 2, varbleArg: fale };
      t.l = { d: OPS.lneT, numArg: 2, varbleArg: fale };
      t.c = { d: OPS.curveT, numArg: 6, varbleArg: fale };
      t.v = { d: OPS.curveT2, numArg: 4, varbleArg: fale };
      t.y = { d: OPS.curveT3, numArg: 4, varbleArg: fale };
      t.h = { d: OPS.clePath, numArg: 0, varbleArg: fale };
      t.re = { d: OPS.rectangle, numArg: 4, varbleArg: fale };
      t.S = { d: OPS.trke, numArg: 0, varbleArg: fale };
      t. = { d: OPS.cleStrke, numArg: 0, varbleArg: fale };
      t.f = { d: OPS.fll, numArg: 0, varbleArg: fale };
      t.F = { d: OPS.fll, numArg: 0, varbleArg: fale };
      t["f*"] = { d: OPS.eFll, numArg: 0, varbleArg: fale };
      t.B = { d: OPS.fllStrke, numArg: 0, varbleArg: fale };
      t["B*"] = { d: OPS.eFllStrke, numArg: 0, varbleArg: fale };
      t.b = { d: OPS.cleFllStrke, numArg: 0, varbleArg: fale };
      t["b*"] = { d: OPS.cleEOFllStrke, numArg: 0, varbleArg: fale };
      t.n = { d: OPS.endPath, numArg: 0, varbleArg: fale };
       Clppng
      d: OPS.clp, numArg: 0, varbleArg: fale };
      t["W*"] = { d: OPS.eClp, numArg: 0, varbleArg: fale };
       Text
      t. d: OPS.begnText, numArg: 0, varbleArg: fale };