Alright, I have done some testing and here's my verdict:
— Sumatra (@sotolf, @iooioio) — really snappy — love it!
— Foxit (@sotolf, @kev) — not open-source, but free and the only one that supports XFA
— muPDF (@yyp) — dead simple (maybe too simple 😅) and doesn't work as good on big PDFs as Sumatra does
— XpdfReader — similar to Sumatra: really snappy and with cleaner (albeit uglier) interface
— Okular (@yyf) — couldn't install since it's only available in MS Store :(
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