While you can go to see who has viewed your Sales Document, you cannot create a list around who has viewed a sales Documents, create a workflow around once someone has viewed a Sales Document or set up custom notifications once a contact has viewed a Sales Document.
Why this is a problem: If a contact owner has changed, the new owner does not receive any notification once their contact has viewed a Sales Document that was sent by the previous owner. You cannot create a workflow that can notify the new owner of the Sales Document view either.
The only person who will get an email notification that a contact has viewed a sales document is the person who originally sent the Sales Document. Previous owners are now forced to find out who the new owner is and forward the notification to the current owner. That's if the current owner is lucky enough to have someone willing to do that work. Also, the previous owner may no longer work for the company which means no one will receive a notification.