Wow, an easy one I can answer.
Open a folder with a file similar to the one you want to set the preferred application -- i.e. pdf or text or html, etc. and then right click on the file; select properties, then click on the 'open with' tab and you can select one of the applications listed. It says to 'open file.pdf
and others like it '... Hopefully the application you prefer is listed, if not you can add. Then close, and it'll save your preference.
I found that when F11 was opening text files with AbiWord and I prefer Gedit.