Honestly, we need to take a hard look at what to do with Evince. I’m not sure. What we should not do is port it to GTK 4 and otherwise leave its current architecture intact. Using poppler in the main process is not safe. We need to push that out to a subprocess that is sandboxed by bwrap (when not running under flatpak) or flatpak-spawn (subsandbox for when running under flatpak). And that sounds hard to do.
Currently our safest way to view PDFs is to use Epiphany, where the sandbox comes for free via WebKitGTK.