.PDF stands for portable document format, hence it is published. Most .PDFs are designed to be printed and filled out, in order to avoid people from tinkering with them like a word document file. Unless there are specific form fields originally created by the person who made the .PDF in the first place.
Did you create the .PDF or are you using somebody else's? If you created the .PDF you can use Adobe Framemaker or Acrobat professional to edit the document. If it is somebody else's, you can try editing it with these programs as well (they are not free, like the Acrobat reader only), you may not be able to edit them though, if they are locked or officially published.
