How do I mark up a PDF on Preview?

I have a PDF pulled up on my Preview application. It will allow me to "annotate" the doent, i.e. put rectangles and circles around things, but I'd like to underline and highlight. However, the options are greyed out under the toolbar, meaning I cannot access any of those functions. Why is this? I have Preview v. 4.2. Do I need to update it, or is it the doent itself?