STEP 1: Click the start button:
STEP 2: Start typing "default" and click "Default Programs" or "Default app settings: as shown below:
STEP 3: Select the Adobe application that you are looking to set. You may have to scroll down on the page to get to the bottom:
(May look slightly different depending on the Windows version but terminology will be the same)
STEP 4: Make sure the desired Adobe PDF viewer is set as the default app next to the file type ".pdf". If not, click on the icon next to ".pdf" and select the correct program.