I got this! Recently discovered a solution trying to help out my mom who was having the same problem.
First off, right click on the document and click "save as" to save it on to your computer (instead of just left clicking it to open it). Next go to where that file is saved on your computer and right click it again, choosing "open with" and then "choose default program..."
Here make sure you have the Adobe Reader option highlighted and click OK, ensuring the option of "always use the selected program to open this kind of file" is checked off as well. This should re-associate your .PDF files to open automatically with Adobe.
HOPE THIS HELPED :3
What is your operating system?
What means "unable" - what happens when you try?
Note that Adobe Reader is a 32-bit applications; it will not run on a 64-bit browser.
I had a V-8 moment as I read your advice/help. Works great...of course, logically!
Thank you very much for the willingness to help! Take care and Happy New Year!