Setting a default program for a particular file type in Windows 8.1:

1. Bring up the Start menu and select "Default Programs"...

... or search for/launch the "Control Panel". In the "Control Panel" click on "Programs", then "Default Programs".

2. Click the "Set your default programs" link.

3. In the "Programs" list on the left side, select (highlight) your desired application (such as Adobe Acrobat Reader),
then click the "Set this program as default" button.