Drag the PDF you need to sign into a new email in Mail. Hover over it to see a small arrow button appear in the top right corner. From here, select Mark Up. The PDF will then open and you'll see a signature option at the top - it's the icon with a squiggle that looks like a signature with a cross to the left.

You can either select a signature you have used in the past, or you can create a new signature. If you choose to create a new signature, you can either use the trackpad on your Mac to draw one, your iPhone, or you can write your signature on a piece of paper and then hold it up to the webcam on your Mac.