Not sure.... I don't know if Acrobat will allow you to annotate on a PDF that was the result of a scanned file. I don't have Acrobat on this PC, can't test it for ya.
If the PDF was natively created in Adobe Acrobat, you can edit it in Acrobat... if it was converted from Word (.doc) all bets are off.
Also - the PDF file can be locked by the originator so the source cannot be edited... this can drive you crazy as well.
Larry, I would ask the originator how they created the document, and if Adobe Acrobat can help you change the form. Most of the time, I re-create the form using word and use CutePDF to create the source. I do have Acrobat and use for documents where I WANT the user to type into the fields using reader, check boxes, etc...
Just because it says .PDF does not mean it can be edited with Acrobat. Also, you do not know just by looking at it with Reader - again, it is a crap shoot when you open it with Acrobat.