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.

for (let i = 0, len = items.length; i < len; i++) {
      if (items[i].str === undefined) {
        if (
          items[i].type === "beginMarkedContentProps" ||
          items[i].type === "beginMarkedContent"
        ) {
          const parent = this._container;
          this._container = document.createElement("span");
          this._container.classList.add("markedContent");
          if (items[i].id !== null) {
            this._container.setAttribute("id", `${items[i].id}`);
          }
          parent.appendChild(this._container);
        } else if (items[i].type === "endMarkedContent") {
          this._container = this._container.parentNode;
        }
        continue;
      }
      this._textContentItemsStr.push(items[i].str);
      appendText(this, items[i], styleCache, this._layoutTextCtx);
    }
  }
  /**
   * @private
   */
  _layoutText(textDiv) {