This issue usually happens when another program updates and associates PDF files to the new program. 

To fix this issue:

1. Right-click on one of your files that used to open up with Adobe Reader.
2. Choose properties from the menu.
3. Click on the "Change" button.
4. Find Adobe Acrobat Reader or Adobe Acrobat DC from the list. OR find Nitro Pro if you have a Nitro license
5. Click OK.

At this point, all your PDF files should now open up in the proper program and your PDF icons should look normal.