New strings landing for PDF Viewer

Hi,
I just pushed a batch of strings to the gecko-strings repositoy, and they
will be available for localization in Pontoon in a few minutes.

I wanted to point out a group of strings that will likely be a challenge to
localize.
https://hg.mozilla.org/mozilla-central/diff/724b88e1831d/browser/locales/en-US/pdfviewer/viewer.properties

These strings introduce a couple of new concepts: "Page spreads" and
"Wrapped scrolling".

For page spreads, you might be able to find references in Microsoft
terminology (in a printing context), but for "wrapped scrolling" it's going
to be harder (only references to "wrapping").

You can find several screenshots and a bit of discussion on the terminology
in this GitHub issue: https://github.com/mozilla/pdf.js/pull/9208

Francesco