Hi. Currently if user open an encrypted PDF file, she/he will not know that this file is encrypted till try to launch annotators tool by: Tools, review (F6) & see that annotation tools not come. I suggest to make Okular showing - beside name of file that appear in title bar at top of program - the word (Encrypted) so that user will instantly know just by opening document that this document is encrypted. This feature existing in most PDF & other document viewers ..... . Best.
This feature till now not implemented though it is very fit with nature of Okular as a viewer. It is useful to have such feature added to Okular, because it is the best PDF viewer (& annotators) on Linux. Moreover, we have no any program - please correct to me if wrong - has such ability on Linux. So it is nice if Okular has it.