Open the document you want to sign in Adobe Reader, and in the right sidebar, select Fill & Sign. Click the Sign yourself button in the toolbar and then Add Signature.
There are a couple of ways you can create a signature: you can type your name and have Adobe Reader convert it into something that looks like a signature, draw one with your trackpad, or upload an image of your signature. Drawing is probably the easiest if you want something unique without too much hassle.
Drag your signature to where you want it to be, save the document, and send it back.