Go to Control Panel\Folder Options\File Types and find .pdf in the list of extensions. Click on it and below the list, you will see the details for that extension. By "opens with" click on the Change button and you should get a dialog asking what program you want to use to open the file.
Pick the program you want [if it's not on the list given, you can browse to the file], and make sure "always use the selected program to open this file" is checked. Click OK on both open windows and that should make your choice the new default program to open .pdf files.