Try opening document in different format or change adobe setting, something with both read and write privilege. Look under "Help" in the menu bar and there should be an option for "About Acrobat/Reader version..." and that will open a pop-up with the major an minor versions of your product.
Signing a form in Reader requires special right have to be assigned to the PDF by the creator of the form and for signatures, this requires the use of a special server.
Did the sender provide any instructions on how to do this?
Maybe they expect you to print out the form, sign it with pen and ink, and then either fax it or scan it into another PDF and email the signed form.
Hope it solves your problem.