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I use Foxit. It's not open source but it can do quite a bit extra.Secure PDF Reader
Have a look at this:
4 Really Light Alternatives to Adobe Reader
I have used Foxit, Sumatra and Adobe. I personally use Foxit because Adobe was too sluggish.
In my experience, there's loads and loads of different PDF readers and ways to read them, but the best, like it or not is Adobe Acrobat Reader