I use a security password, also know as a "document password" to prevent sold documents from being easily re-distributed.
The document password is unique to each document and is required to open it.
This is a common security feature used on Adobe PDF documents.
Document Seller does not seem to support the use of document passwords yet it would be an easy fix.
Under Document Management, for each document, just add a field to enter the password.
Then when a user buys a document, they would receive the password for that specific document in the order confirmation email. A new tag called [PASSWORD] would be required.
The admin would then edit the order confirmation message as: The security password for your document is: [PASSWORD]
This is a relatively simple feature.
Thanks!