@dibl,
a few more ideas:
- open dolphin (okular) with the terminal and run the whole procedure. Watch the terminal, maybe it tells you errors
That is an excellent suggestion -- I should have thought of it myself.
So, I did that and I got two surprises:
1. A random PDF opened in Okular as expected (the problem is with all PDFs, regardless of the quality).
2. I saw this surprising list of problems in the terminal window:
kf5.kio.core: "Could not enter folder tags:/."
inotify_add_watch(/lost+found) failed: (Permission denied)
inotify_add_watch(/root) failed: (Permission denied)
inotify_add_watch(/run/alsa) failed: (Permission denied)
inotify_add_watch(/run/cryptsetup) failed: (Permission denied)
inotify_add_watch(/run/initramfs) failed: (Permission denied)
inotify_add_watch(/run/lvm) failed: (Permission denied)
inotify_add_watch(/run/openvpn-client) failed: (Permission denied)
inotify_add_watch(/run/openvpn-server) failed: (Permission denied)
inotify_add_watch(/run/sudo) failed: (Permission denied)
inotify_add_watch(/run/udisks2) failed: (Permission denied)
discarding "Send SMS via KDE Connect" ("ShareUrl")
These docs are on an luks1 encrypted file, which I open with zulucrypt and dolphin before doing any work. I wonder if that is related to all the "permission denied" messages since, for this test, I ran dolphin as a user from my terminal.
As a further test, in dolphin I browsed to my downloads folder where there are also some PDFs. They open in Okular as expected with one click. So, this tells me that the issue with the PDFs on the encrypted file is related to encryption.
Thanks for the help!
I will mark this as solved since I now understand where the issue is, and expect that it will be worked out in further development of the packages involved.