How to Sign any PDF that hasn't been Prepared through Adobe Sign (video)
Note: This video is about signing 'normal' PDF files that happen to have signature lines (such as a PDF a colleague emails to you). To sign documents sent through Adobe Sign, see this video.
What's the difference? Documents sent through Adobe Sign will arrive in a special Adobe Sign email that contains a link to a document, which will contain clear instructions about where to sign. They also automatically generate confirmation emails to senders and signers.