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Adobe Acrobat Reader - the classic choice, free and will open any PDF, but editing features are pretty limited there. If you need proper editing functionality, check out Foxit Reader (great balance of features and speed) or PDF-XChange Editor - you can add text, delete objects, add signatures, work with forms. For more serious tasks there's paid Adobe Acrobat Pro, but for typical document work one of the free options will do the job.

I used the Windows viewer for a long time - the one built into Windows works without any fuss, but when I needed to edit, I switched to Foxit. Annotations are more convenient there, and filling out forms is simpler than in Adobe Acrobat Reader.

If you need serious editing - adding text, changing content - then Foxit is just okay, you're better off looking at full-featured editors like Nitro or LibreOffice Draw, but they're heavier.

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