I know how to set default web, mail, calendar, music, video and photo-viewing software (by opening System Settings) but I'd like to be able to set my default PDF viewer.
Right click on any pdf file and choose Properties. Under the 'open with' tab, you will find the applications to open PDF files. Choose your application of choice and then press the 'set as default' button in the right bottom corner.
See screenshot below:
Right click on any pdf file, in the menu select "Open with...", then select the app that you like to open pdf from the list. Now clicking on any PDF file it will be open in the selected app.