How can I make another pdf viewer standard for pdf files in Ubuntu?
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..
right click on a pdf file, click on "Properties", go to "Open With" tab and select the program you want. You can change the default program for any kind of file using this method. Here is a similar question: How can I change the default program for viewing pictures and photos?