Adobe Acrobat 7 Professional - Manual (Page 330)

Viewing previous versions of a signed document
If a document is signed more than once, all the signed versions are maintained in a single
Adobe PDF file. Each version is saved as append-only so that it cannot be modified. All
signatures and their corresponding versions appear in the Signatures tab.
To view a previous signed version:
Select the signature in the Signatures tab, and choose View Signed Version from the
Option menu.
The previous version opens in a new Adobe PDF file, with the version information and
the name of the signer in the title bar. To return to the original document, choose the
document name from the Window menu.