iOS: We've talked about signing documents from your phone before, but one of the lesser celebrated features of iOS 9 was integrating this directly into the Mail app.
iOS 9 brought along "markup" to Mail, which allows you to draw on images and add notes. Let's be honest, this is a feature that most of us probably never use. Tucked away in that menu is also a signature section where you can easily sign a PDF and send it off. Just open up a PDF file in Mail, tap the toolbox icon, then tap the signature icon in the bottom right corner. Use your finger to sign it, and you can mail it back to the sender. It's not a feature most of us will need very often, but it's nice to know it's in there.