KDE 15.08.3 is in use with Krusader from current git. On hitting enter on pdf file inside zip archive Okular shows error "Could not open krarc:/some/path/to.zip/file.pdf. Reason: Could not start process Unable to create io-slave: klauncher said: Unknown protocol 'krarc'." F3 opens the file with lister in hex mode.
The same with KDE 15.12.0.
Yes, it is correct; as Okular doesn't use KIO::KrArc, this is an unknown protocol. The solution is to decompress the file into a temporary folder and open it from there. Meanwhile, you have to do it manually.
Since Okular hasn't been ported yet to Plasma, Krusader cannot load its part, like with Kate or Gwenview. When Okular will be released, this bug will be automatically fixed. I haven't find any info about the timeline on https://mail.kde.org/mailman/listinfo/okular-devel
I have tried okular-frameworks-git from Arch Linux AUR - yes, the issue is gone with it.