I believe I know what the problem is. You have few applications that can open PDF files but you never chose preferred one therefore system choose semi-random one for you (the most probably in alphabetic order which is obviously gimp, not okular). Just right-click on the file, then click on Properties in the context menu, and choose Okular in the 'Open with' list then close that Properties window. I think this will fix your issue. Let us know if something is still wrong. Thank you very much.
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