As of a few minutes ago, i merged the code from Chinmoy Ranjan Pradhan's GSOC to support showing PDF Signatures and Certificates in Okular.
Signature handling is a big step for us, but it's also very complex, so i expect it to have bugs and things that can be improved so testers more than welcome.
Compiling is a bit "hard" since it requires poppler 0.73 that was released a few days ago.
But thanks to flatpak, there's no need to compile it, you can run the KDE Okular Nightly on your system to try it
flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
flatpak remote-add --if-not-exists kdeapps --from https://distribute.kde.org/kdeapps.flatpakrepo
flatpak install kdeapps org.kde.okular
Note: if you have okular installed from another flatpak repo (for example flathub) this will switch you to the KDE Nightlies, you may want to switch back after testing.
And then you can try the adobe sample pdf
flatpak run --share=network org.kde.okular https://blogs.adobe.com/security/SampleSignedPDFDocument.pdf
And you should get stuff like this