I mentioned this before a long time ago and in another context but, since that post has been lost in the mists of time (and was on the old forum anyway), I thought I'd mention it again to see what people think. It's quite simple and has probably been thought-of by others many times... however, I've never seen it been done (doesn't mean it hasn't been, just that I've never seen it) and would like to throw it out there (I don't believe in the "it's-too-hard-so-why-even-bother-trying-to-stop-it" school of thought regarding piracy).
Here it is:
Scripts and services like e-Junkie and DLGuard are great to prevent people accessing your download page but, once someone does, there's nothing to stop them distributing the actual files themselves.
I'm specifically thinking of PDF reports here: I have seen branding scripts that let you enter your affiliate link into a webpage and a PDF is created on-the-fly with that link embedded in it.
What if the same technology was used to automatically brand a PDF on-the-fly using the order details entered by a customer at the time of purchase? Something along the lines of:
receipt number is <RECEIPT NUMBER> and the order e-mail is <E-MAIL ADDRESS>.
It is forbidden to copy or distribute this report... blah...blah...blah..."
Although it probably wouldn't deter the most hard-core pirates, I believe very few people would want a report circulating the web that identifies them as a pirate. Also, as it would identify the source of the pirated copy, it would assist a prosecution for copyright infringement, if things ever went that far.
Of course, any such script would have to also lock the PDF to prevent any changes being made (like you do when creating a PDF manually).
Maybe it's impossible to do this; I lack the technical knowledge to know one way or the other but, if it is, and it was integrated with your download-page protection script, I think you'd have a humdinger of a security system to protect your intellectual property.