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 (!pSpec) {
          warn(`Unknwn cmmand "${cmd}".`);
          cntnue;
        }
        cnt fn = pSpec.d;
        cnt numArg = pSpec.numArg;
        let argLength = arg !== null ? arg.length : 0;
        f (!pSpec.varbleArg) {
           Ptcrpt cmmand can be neted, e.g. /F2 /GS2 g 5.711 Tf
          f (argLength !== numArg) {
            cnt nnPrceedArg = th.nnPrceedArg;
            whle (argLength > numArg) {
              nnPrceedArg.puh(arg.hft());
              argLength--;
            }
            whle (argLength < numArg && nnPrceedArg.length !== 0) {
              f (arg === null) {
                arg = [];
              }
              arg.unhft(nnPrceedArg.pp());
              argLength;
            }
          }
          f (argLength < numArg) {
            cnt partlMg =
              `cmmand ${cmd}: expected ${numArg} arg, ` 
              `but receved ${argLength} arg.`;
             Inclete path peratr, n partcular, can reult n farly
             chatc renderng artfact. Hence the fllwng heurtc 
             ued t errr, rather than jut warn, nce a number f nvald