You can sign PDF documents in the Preview app and in Mail. You save your signature using either the trackpad or your Mac's camera. You can then insert that signature in any spot in the PDF. It then becomes a permanent part of that PDF just as if you had printed out the document, signed it, and then scanned it back in. You can do the same in Mail, but only the copy sent will contain the signature.
Related Subjects: Mail (84 videos), Preview (43 videos)