I've setup okular signing and it worked, but I believe it was with a mime certificate tied to my email (and not pgp keys). If you want I can try to figure out exactly what I did to make it work.
Briefly off the top of my head, I believe it was
- Getting a mime certificate for my email from an authority that provides them. There's one Italian company that will do this for any email for free.
- Converting the mime certificate to some other format
- Importing the certificate to Thunderbird's (or maybe it was Firefox's) certificate store (and as a sidequest setting up Thunderbird to sign email with that certificate
- Telling Okular to use the Thunderbird/Firefox certificate store as the place to find certificates
I can't remember if there was a way to do this with pgp certificates easily
I'm sure they'd welcome a pull improving the UX! https://invent.kde.org/network/kdeconnect-kde I think the implementation of the protocol is pretty well isolated from the UI, so pretty radical UI changes should be relatively easy