Acrobat users can comment on PDF forms, just as on any other PDF. If the form creator has extended rights to Adobe Reader users, they can also add comments.
Whether or not these comments are included when the form is submitted depends on how it’s submitted. For example, if you use Reader to print the form for mailing or faxing, the comments don’t appear. If you attach the filled-in form to email as a complete PDF, the comments are included.