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Why does the "set sumatra as defaul" not work?
Because it is a portableapp you cant set it as the default. If you want to use Sumatra as a defult sownload portable file associator or CAFE Mod/Espresso and thru them you can set it up as the default and anyother app as your default with out messing up the portability of said app.
For better info on this refer to the suggested app's threads. Hope this was helpful.