Skitch for Mac and iOS is an excellent app that I've used for creating annotated screenshots for quite a while. In fact, I used it to create the screenshots in A Short Guide to Using Google Drive on Your iPad. Recently, Skitch for Mac and iOS received some updates that make annotating PDFs easier and better than ever before.
Skitch now allows you to import and annotated single and multiple page PDFs. New stickers and call-outs have been added to Skitch as well. If you're an Evernote Premium account holder you'll have access to even more features including an annotations summary page.