Once a form has been signed with a digital signature, why can't you insert a bookmark?

Question

We are using Adobe 9 on Windows XP, I'm not sure what the forms are created in.
I understand why you wouldn't allow a form to be changed once it's been signed digitally (because the person is attesting to the contents as they are when the person signed the form). I don't understand why we can't add bookmarks to pages in the form? The bookmark technically is a change but not a substantive change to the form. Is there a way to add links to places in the form after a digital signature has been created?

Answer 1

Hi Christic3,
This is a bit geeky, but here goes. Think of the PDF file in two layers (there are really more, but we'll keep it at two for simplicity). The bottom layer is the PDF content such as fonts, structure tags, and pages. The top layer is form data and comments (aka annotations). When you sign the PDF you sign everything (both layers), but the only allowable changes to the file are additions to the top layer. You can add form data and comments, but you cannot modify anything in the bottom layer without breaking existing signatures. In fact, you can't change (by change I mean modify or delete) anything on the top layer either, but you can add new (unsigned) items.
Bookmarks are part of the underlying PDF structure. Because they are as integral to the underlying structure as anything else, the ability to modify the bookmarks is locked after signing, otherwise it would be too easy to break the signature.
Steve