It depends on your needs I suppose.. For simple PDF opening/saving/reading any program will do, even web browsers can open them directly these days. I use Adobe Acrobat Reader, another good lightweight is Javelin PDF Reader.
As a bigger suite that can fill out and save forms, edit/annotate PDFs, etc. the most popular two are Adobe Acrobat and Foxit I think.
I've heard good things about Sumatra PDF, it is apparently fast and can also read ebook formats, but it does not support forms, I haven't tried it personally.
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