Gave evince (gnome document viewer) a try (1/2)
Purism gives a "fully optimized" rating on https://tracker.pureos.net/w/pureos/mobile_optimized_apps/ so … high hopes.
For work, I sometimes read these https://csrc.nist.gov/publications/sp800 so that’ll be my real-world test case.
I downloaded it from the gnome browser w/o trouble.
In portrait, it worked well. Rendered fine. Pinch-zoom fine. Full-screen fine. Responsive.
About as good as you can expect for reading a pdf on a phone.