A great new feature of Acrobat 6 is the ability to enlarge and reduce the size of Thumbnails. Adobe decided to changed the name of Thumbnails to 'Pages' - so when you want to see them you'll find them in the 'View > Navigation Tabs > Pages'.
Although it was possible to attach non-PDF file types to your PDF document in version 5, Acrobat 6 has taken the concept a little further and given you a complete control centre to manage File attachments.
A new feature with Acrobat 6 is the Snapshot. A Snapshot is an area of a PDF page or the whole page that you define with the SnapShot tool that becomes a destination for use in actions (links, bookmarks etc).
A nice new feature of Acrobat 6 is the ability to 'lock' in the settings for a particular link. Now this might not seem immediately useful, but when you have to duplicate links to lots of pages and don't want the link boxes to be easily moved by other Acrobat authors -- then this feature is for you.
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.