Custom zoom + "Zoom text only" breaks pdfjs

Original report: https://github.com/mullvad/mullvad-browser/issues/368

Firefox says that "Zoom text only" might break with a default zoom that isn't 100%, but for some reason pdfjs has problems with Tor Browser but no problems on Firefox.

Upstream Bug where they added the warning about breakage: https://bugzilla.mozilla.org/show_bug.cgi?id=1864445.