Step One: Click the Start button in the bottom left.
Step Two: Type in "Default Apps" and Select the first option "Default Apps".
Step Three: At the bottom of the Default apps area, select "Set defaults by app".
Step Four: In the Set defaults by app area, select "Adobe Reader" and click on Manage.
Step Five: Select any file association and update to Adobe Acrobat where needed.
Your are done! .pdf files should open by default in Adobe Reader.