To edit a PDF, start by opening it in Foxit instead of Reader. Next, click Comments, Typewriter Tools, Typewriter Tool. Now just click wherever you want to add text, then start typing. (The program offers other markup tools as well, in case you're interested.) When you're done, you can save, print, or e-mail your marked-up document.

Foxit Reader is free, but be sure to decline any extra toolbars or the like it tries to sneak in during installation.