I am sure this is a coincidence...
I downloaded Okular and ran the standalone under Sandboxie (which is what I normally do to avoid polluting my PC with stuff I may not like, but allowing it to work in a real environment). Seemed OK but Okular didn't seem to bring anything to my party, so removed it. However, via a circuitous route, I found out it could display markdown properly - most other markdown-aware things display it as raw weird stuff with the proper display as a separate 'preview', so this was attractive. Accordingly I then ran the installer version of Okular.
Didn't seem to work right and complained that it couldn't write to some file because of permissions, and then Bitdefender popped up and killed it. WTF! Long story short, my TheJournal executable was banished from my PC, Miranda and Waterfox lost their profiles and other bad things happened.
Seemed to be a registry delete-fest too. Fortunately, blasting back my C drive en bloc from an image sorted it all out, and again running the standalone Okular in a sandbox was fine. As was running the installer version in a VM.
Highly unlikely Okular downloaded from KDE is malware (which was the Bitdefender report), but in several years that's the first time Bitdefender has twisted its knickers so messily. I am putting it down to coincidence but thought it worth mentioning if it encourages people to ensure they have regular backups.