When you open a PDF in Preview, you should see a little toolbox - clicking on it will open an editing toolbar where you can insert text boxes, shapes, and even your signature! It looks like this:
Since Preview is a device-based program, just be sure to SAVE so your revisions will stick.
if you are on an iOS device like an iPad, I like Tiny PDF. It allows you to highlight, underline, add stickers . . . almost everything that Preview does for the computer.