By default Okular has the menu bar visible. If you hide it, the toolbar will sprout a hamburger menu. The "Digitally Sign" menu item is quite buried in this menu structure; I suspect most users will never find it. When using the hamburger menu UX, it would probably be good to surface this important action higher in the menu structure--possibly even at the root level of the menu.
A possibly relevant merge request was started @ https://invent.kde.org/graphics/okular/-/merge_requests/758
Git commit c1165fba5dd7cf1e4b3e3377779051969717f766 by Nate Graham. Committed on 15/06/2023 at 05:12. Pushed by ngraham into branch 'master'. Surface Digitally Sign action more obviously in hamburger menu This is an important feature of Okular, but when using the KHamburgerMenu UX, it's quite buried in the More... section. Let's move it up to the top level of the menu. FIXED-IN: 23.08 M +2 -0 part/part.cpp https://invent.kde.org/graphics/okular/-/commit/c1165fba5dd7cf1e4b3e3377779051969717f766