When you open the PDF document w/ Acrobat Reader, proceed as follows:
Navigate your cursor to ...
File menu\ Document Info\ Security option
What does it show for "allowed" permissions? "Changing the document" and "Selecting text & fields" should be set to allowed. PM me your email and I can send you another PDF file to try out, but make sure you check this one first to see the security settings the author placed on the file.