The question has been asked about where in Linux to select a default program to open a given file type (e.g., PDF files). A better question might be where in a particular desktop environment to set this up. Without further ado...
In KDE, the simplest way I know of to set this up is to open Dolphin, navigate to a file of the type under consideration, then right-click the file, select "Open With", and then "Other". When the dialog box opens, either navigate through the menu tree given and select or enter the full path to the application you would like to use as the default in the text box at top, then check the box at lower left that says, "Remember application association for files of this type" and KDE will thenceforth remember how you want these files opened.
Hope this helps get it done for ya.