Use legacy mode of pdfjs v4 for compatibility with iPad Safari and older browsers

!487 (merged) updated pdfjs v3 to v4, and because of the 5 year gap from the previous version, it seems that we now need to use a legacy build to support older or "unsupported" browsers, which includes Safari.

I don't have an iPad device where I saw the problem though, so I need someone to check that they can't see any PDFs in the current prod, and that it's fixed in this MR.