One interesting new feature of OS X Yosemite is the ability to add annotations to images and PDFs in email. This allows you to sign PDFs by simply attaching them to your mail messages and marking them up from there. You can also add text or highlights, or draw directly on the PDF.
1. Compose a new message or reply to an existing one in the Mac Mail client.
2. Attach a PDF file to the message.
3. Click on the attachment and select Markup from the menu which appears.
4. Add your notes or annotations and click “Done.” You can even add your signature, if you’ve saved one (How to save your signature on a Mac).