In Gnome you can set a PDF file to be opened with Okular by right clicking it and choosing: "properties > open with > Okular" but this particular PDF...
If the Flatpaks works like the Snaps, you stay in the same branch* that you installed from - in your case, the beta repo, I think. Snaps however will...
Good to know. Once the fix makes its way into the stable release, I would advise removing the beta version and installing the stable release, unless...
I tried gliv like you suggested and it works fine, it uses about 150MB of memory and the xorg-service uses 30MB. Shotwell and gThumb also works fine....
