As reported elsewhere [1], neon always ships the newest version of Okular, that already support digitally signing PDF documents. As Okular shipped by neon is built against old version of Poppler (currently 0.86.1) , the option is not available (Tools>Digitally sign is greyed out). Current workaround is to use packages from flathub, which comes with other problems specific to the flathub (slower startup times, filesystem access). Having two installations of the same application, also means that one needs to remember which option in Open with menu to select before opening a document for signing, which breaks user experience. [1] https://www.reddit.com/r/kde/comments/mzp3vs/okular_2104_there_are_no_available_signing/
*** This bug has been marked as a duplicate of bug 446309 ***