OS X’s Preview.app is a great PDF reader but not a very good PDF editor. It doesn’t support pasting images into PDFs, so when I’ve had to add my signature to a PDF in the past, I would drop the PDF into Acorn (and previously Pixelmator) and paste away.
My main problem with this approach is that Acorn (and Pixelmator) rasterizes the text in the PDF, resulting in blurry text and a resulting PDF that no longer has selectable text or form fields.
Today I discovered PDFescape, an online PDF editor which allowed me to upload a PDF, paste my signature into it, and download the new file — complete with selectable text and form fields. I highly recommend it for any PDF editing that Preview.app can’t handle.