This env: ‘/snap/bin/firefox’: Permission denied or env: ‘/snap/bin/chromium’: Permission denied issue in evince launched through a terminal can be traced back to apparmor. Doing tail -f /var/log/syslog | grep -i apparmor (from here), you may find a line where the following appears apparmor="DENIED" operation="exec" name="/usr/bin/snap" profile="/usr/bin/evince" comm="env".
This can maybe be fixed by tuning the /etc/apparmor.d/usr.bin.evince profile, or by a modification of the abstractions like /etc/apparmor.d/abstractions/ubuntu-helpers or /etc/apparmor.d/abstractions/ubuntu-browsers (e.g. here or there).
A radical change which worked for me but may lead to security issues is to just disable apparmor for evince with sudo ln -s /etc/apparmor.d/usr.bin.evince /etc/apparmor.d/disable/usr.bin.evince. It it written there that sudo service apparmor restart would then engage the changes but a full computer restart was needed in my case.
EDIT: A somewhat less radical move could be to add a /usr/bin/snap Ux, line towards the end of the /usr/bin/evince {...} block of /etc/apparmor.d/usr.bin.evince (see here) and restarting the service with sudo service apparmor restart. That may still not be entirely safe, though.
