STEP 1: Click the start button:
STEP 2: Start typing "default programs" and click the icon with the gear as shown below:
STEP 3: Select "Set defaults by app" as circled below. You may have to scroll down on the page to get to the bottom:
STEP 4: The following window will appear, select Adobe Reader DC and the choose "set this program as default" as highlighted in yellow below:
Click "OK" and PDF files will now open in Reader DC Pro.