Hi,
My name is Angel. I am an Independent Advisor. Thank you for posting in Microsoft Community.
I would recommend that you review the following guide:
https://ccm.net/faq/20348-sumatrapdf-set-as-def...
If issue is still not solved:
1.In file explorer, right-click any PDF > Open with > Choose another app .
2.Check the _Always use this app to open .pdf files _ checkbox.
3.Scroll down and click on the More apps ↓ link, then select SumatraPDF from the list and click on the OK button.
4.If SumatraPDF is not in the list, scroll down and click on the Look for another app on this PC link.
5.In the Open with… dialog navigate to c:\Program Files\SumatraPDF (or c:\Program Files (x86)\SumatraPDF ), select **SumatraPDF.exe ** and click on the Open button.
6.The PDF should open with Sumatra, and all PDFs when subsequently double-clicked should open with Sumatra too.
Disclaimer: There are links to non-Microsoft websites. The pages appear to be providing accurate, safe information. Watch out for ads on the sites that may advertise products frequently classified as a PUP (Potentially Unwanted Products). Thoroughly research any product advertised on the sites before you decide to download and install it.
Hope the information provided is useful. If the issue persists, reply here and we will be glad to help you.
Angel.