Where can I set the default programs?
Solution 1:
Open up System Settings by searching for it in the Activities view, then go to "System Info" (in the bottom row).
Here you can select Default Applications and pick your favourites.
Solution 2:
Unfortunately you can't change the standard for opening pdfs there.
For that you can right click on a pdf file, choose 'Properties -> Open With', and then 'Make default' (someting like that, I use other than english lang ;)).