The below steps will show you how to set adobe reader as a default app for pdf files:

Step 1: Open Adobe reader and you should get the below message, if you get the message, then go to step 3, if not perform step 2.

Step 2: Click on Edit> then click on Preferences>> then click on General Tab>> then scroll all the way down to "Application startup" section and click on "Select as Default PDF Handler".

Step 3: Click on "Continue"

Step 4: Under Open with click on change and select Adobe Reader then click on OK twice.