I recall using SumatraPDF in its early days. I also worked at the IT department for a university and we started putting it in all PCs. It was fast, reliable and no licensing issues whatsoever.

I use zathura on Linux nowadays, and it like a not-too-distant cousin. It's perhaphs a bit less newbie-friendly, but it shares the same focus on "let's just render things fast and no BS", and also uses mupdf under the hood.