It just happened again, I open a PDF file using File/Open function and Okular would suddenly open up the file in full screen mode. In this case it was the second PDF I tried to open. The second file was opened in a new instance of Okular, but in full screen mode. I had previously noticed that Okular would suddenly open a PDF file in full screen mode even when there is no other previously opened file. What is even more bothering that the escape key does not work. There is no right click menu or any other obvious way to return to normal mode from full screen mode. Very annoying.
You mean presentation mode or full screen mode. They are different. (Full screen is basically regular mode but with less window decoration). Full screen mode should be impossible. Presentation mode warns you what you have to do to exit presentation mode, unless you pressed the "I know what i'm doing don't show me this dialog again). If you open that second PDF you try to open all by itself, does it also go to presentation/full screen mode?
If you can provide the information requested in comment #1, please add it.
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I am happy to report that this issue did not occur anymore since I reported it on 9/1/2018! :-)