SumatraPDF registers itself for all supported file types (.pdf, .png etc.).
If there are no other applications registered for a given extension, you can open the file in SumatraPDF simply by double-clicking the file in File Explorer.
If there is more than one application, you have to explicitly tell Windows 10 to make SumatraPDF default application.
In File Explorer:
Open withand then
Choose another app
From the list, choose
SumatraPDF and check
Always use this app to open .pdf files:
This is based on latest Windows 11 build at the time of this writing.
Unfortunately the details differ between Windows updates.
Default app section in settings app, e.g. use
Windows logo hot-key to launch system-wide search, type
default apps and click on
Default apps search result to launch settings app.
In Default apps type
Click on current default PDF application (
Microsoft Edge in this example) and select