If you want to annotate a PDF on a Mac there are a bunch of ways you can do it. Heck, you can even use the built-in Preview app to do common things like draw boxes around areas or type text anywhere you want. If you are constantly working with PDF forms there is a better option that I like and it is called FormulatePro.
With FormulatePro you only get a handful of editing options including shapes (circle or square), freehand lines, text, and checkmarks. I really like that they added in a checkmark option because in a single click you can check a box on a form, which if you are doing your taxes the old fashioned way by filling out the forms you can try using this app… because there are plenty of boxes you’ll be checking. And then once you’re all done you can export the edited form as a PDF.
ForumlatePro Homepage (Mac only; Freeware)