One underrated feature of PDF is its ability to act as a container for other files. One example of where this could be useful would be attaching the original Word, Excel or other source files to the final PDF document, allowing it to be more readily updated. Since it's possible to attach literally any type of file, the PDF can also act as a secure delivery medium for sensitive or commercial content.
Here's how to add a file attachment to your PDF file in Acrobat 7:
Open Acrobat and select Document > Attach a File.
Browse for the file that you wish to attach.
Activate the 'Attachments' pane by selecting View > Navigation Tabs > Attachments, or by selecting the 'Attachments' tab from the 'Navigation' pane (if this is already active).
OK, so you want to stamp your document. Maybe you need to give reviewers some advice about the document's status or sensitivity. This tip from author Ted Padova demonstrates how to add stamps with the Stamp Tool along with related comments.