If you are already a Skitch user and happen to have an Evernote Premium account, a new update now allows you to annotate PDFs using the app. Once you open a PDF in Skitch you'll get a 30-day trial of the premium PDF annotation feature. All users see a new way to add graphics to documents in Skitch: stamps. Like map markers, these can be used to call out specific points with an icon and text for easy viewing. There are many icons available for call outs, like an exclamation point or heart, and you can add text next to the stamp.
Skitch has always been a handy tool for quickly marking up an image for collaboration, but the PDF features add to Evernote's suite of tools for managing, storing and now altering documents. Other PDF improvements include a text notation tool, plus all the shapes you could add to images. There's a summary feature of annotations for PDFs, which adds a page to a PDF you share, cleverly pulling out the marked sections and displaying them in a summary page. I can see this saving a lot of time when sending and marking up large documents. Again, any PDF over 25 MB will require a Premium account.
Skitch is a free app but Evernote Premium is $5 a month or $45 a year.