Put the mouse pointer on the .pdf file, right click and see if you have the "open with" option. If you do not, hit the escape key and do this over again holding down the shift key. You should then have the "open with" option.
Highlight "open with" and it should give you some choices... chose Adobe Reader if it is there. If not, click on "chose program" and then pick Adobe reader if it is in the choice list. If it is NOT in the list, you may have to chose to locate the .exe manually. Do so. Make sure that the check box at the bottom of the dialogue box "Always use this program with this kind of file" if that is what you want to happen.