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Rodney is totally right - it's a mission impossible to achieve that.
Just look at
http://www.slideshare.net - some of the presentations "blocked" to download. But you can still take a screenshot of each of them and and re-assemble into a presentation.
The best way to protect the content is to incorporate a "watermark", as Rodney said, but with the name of the viewer into each page they view. Of course, you couldn't use cache for any of these documents
Then, even if they do a screenshot, it would be very hard for them to remove their name without damaging the content, and they would be concerned that you could trace them.
Going step further, you could also incorporate a second
watermark or fingerprint with their name, but in an invisible way, and make them aware of it. Something like a hologram, that you could only see with a special reader
This way could you have a proof who downloaded it.
But then again - would it be worth all the hassle?