How can I make another pdf viewer default for pdf files?
Solution 1:
You can right-click on a .pdf file, then go to 'Properties' --> 'Open with' tab and select the application that you want.
Solution 2:
you can also used this simple utility ubuntu tweak which you can use for any application ubuntu tweak. You can use it to change the music player for any mime-type. after installing go to filemanager..