I'm trying to create a pdf that will allow multiple people\agencies to review and sign-off on a document. Can I create a pdf in Acrobat 8 Professional to allow for multiple people to sign the document, show their signatures and if someone makes any changes, the previous signatures are invalidated? :/
If so, are there any "How to" guides?
The trick comes in when you talk about "make any changes" and wanting the previous signatures invalidated.
When we talk about "changes" to a PDF file, there are several kinds of changes possible. The effect of the different kinds of changes depends on how the document is set up.
First, if the bits of the file get modified (whether through deliberate action or otherwise), any signature that covers those bits will become invalid.
But other changes are trickier. Suppose you sign a document. Then someone else signs it. Isn't that a "change"? Technically, it is. But its pretty certain you don't want the addition of a signature to invalidate earlier signatures. But there are other changes that are less desirable.
The best approach to this is a "certified document" that allows form fill-in and digital signatures only. Ideally, each form field would be associated with one signature field, such that until that signature is applied, the form field could change without invalidating prior signatures. Once that signature is applied, any changes to that form field would invalidate that signature. And if anyone make changes other than filling in a form field or signing a signature field, the entire document becomes "invalid".
For details on this, see the various documents on http://www.adobe.com/devnet/acrobat/security.php I'm sorry that I'm not more familiar with all the available resources to be able to point you to a single "How To" guide. But I hope I've given you some ideas and leads.