Appending new pages to signed documents now invalidates signature

Hello!

I have developed a service that appends additional pages to existing PDF documents. Some of the documents the service works on happen to be signed. As the service appends the pages using incremental updates, the signed contents are left untouched. This makes Acrobat 8 and Reader 8 display the signature state as "valid with subsequent changes" which is ok and no one complains about.

Now along come Acrobat 9 and Reader 9, and all of a sudden the signature state is "invalid" which my users do complain about. Telling them that inspecting the signature state details they can see that the signed content still is untouched and the actual change is recognized to be some appended pages, doesn't help either as they in turn forward the documents to their customers who don't accept the documents if the Adobe Reader sais the signatures are invalid.

Does anyone know a way new pages can be added to a signed document without having the new Adobe program versions choke on them?

Even a hint that involves adding some flag to the original unsigned document or the signature itself would help as the users in question can tweak those processes. Due to the nature of the workflow, though, signing unfortunately must happen before my service can add the new pages.

Best regards, Mikel.

By the way, the signatures in question are regular approval signatures, nothing fancy, especially not certifying signatures (which indeed can forbid any changes or at least any changes but some form fill-ins and annotations)...