I was using an older version of Adobe and couldn't type onto a PDF. It popped up a note online that told me to upload a new version, which I did (9.2).
I still can't type anything. The cursor is there but when I type, nothing happens. It doesn't move. I can go to "tools" and click on the hand tool or arrow, and those don't give me a response, either.
To be able to type on a PDF, it either has to have form fields or you have to add them (or you can use the typewriter).
Do you have Acrobat or do you have Reader? If it's Acrobat, use the form field tools. If Reader, you are out of luck unless it contains those fields (which it doesn't sound like it does).