If okular does not find a file when started with a command line argument, it displays an error message to indicate the error (which is completely fine).
Clicking "Ok" closes this message and leaves an open, empty okular window.

This error may happen because of a temporary unavailable resource (e.g. if the directory that contains the file is not mounted).
When restarting kde/plasma with several open okular session this can be quite annoying, since it produces many open, but empty, okular windows and the error messages. 
To open the files again, one needs to browse to each of the files location. 

Therefore, it would be great if pressing "F5" on such a window (okular session that failed to open the provided file) would try to load the file again from the same resource.

Some other kde applications (e.g. kate) show this behavior.