I am new to this so don't be surprised if this sounds like a dumb question so be gentle. When I click on a PDF file in Firefox it either asks me to choose an application or it opens in GIMP which works, but I'd really rather use Okular (or some such program). I don't know how to either a) add Okular to the list of applications to select in the preferences menu, or b) choose Okular from the 'choose applications' dialog box. I think this might simply be a question of being able to locate the path of the program.
Incidentally, program files in Linux are obviously not .exe files. What is the file extension for these programs ... I did mention I'm new to Linux, so go easy on me.