STEP 1: Click the start button:
STEP 2: Start typing "default" and click "Default Programs" or "Default app settings: 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:
(May look slightly different depending on the Windows version but terminology will be the same)
STEP 4: The following window will appear, select Adobe Reader DC and the choose "set this program as default" as highlighted in yellow below:
(May look slightly different depending on the Windows version. You may need to click "Manage" after selecting the app. If you see Manage, scroll to the last picture.***)
Click "OK" and PDF files will now open in Reader DC Pro.
***If you saw "Manage" instead, you will see the following screen.
Make sure Adobe Acrobat DC is set as the default app. If not, click on the icon next to ".pdf" and select Acrobat DC.