Dan Secure Document Dilemma
If you were to go with an electronic medium like PDF, you could resonably protect the document from being printed and having text extracted from it (via AES encryption and a long password). However, you couldn't protect the document and it's view/open password from being shared amongst individuals wishing to gain access to it.
With a hardcopy, there is nothing you can do to physically stop someone from photocopying or scanning the document into a tradeable PDF file. However, I would highly suggest customizing EACH copy of the document you send to your customers with multiple identifying factors, and keeping a copy of each under their file. These customizations can include watermarks, borders, fonts, footnotes, paragraph spacing/order, etc. This way, if the document does happen to leak, you can trace it to a specific individual.