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The obvious choice is one of the various Adobe tools.

If you're feeling in a not-spend-money mood, you can do it with ImageMagick.

http://www.imagemagick.org/Usage/annotating/

Writing the command lines for it takes some getting used to, but after you do it you can do some VERY fun things with ImageMagick. I'm using it to do a lot of automated conversion/cropping/etc for the next version of my app.

convert some.pdf some.gif

It is really magic.

You can save individual pages from a PDF as an image in Preview (scroll to the page, File -> Save As). This won't work if the PDF is protected but that would likely apply to other applications too.

You can also create PDFs from images with Preview. Open an image and save it as a PDF. You can then drag images or pdfs into the sidebar which adds pages to the PDF.

It's not ideal, but it works.

As an alternative you could e-sign the document using a service like EchoSign. While most people use it to send out documents for e-signature, it also has a feature called approve that lets you e-sign documents people have sent you.

Ask HN: PDF Signatures