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davidcyclebackFor the publisher, the key to security is to have the password be the purchaser's name and social security number. The purchaser won't be giving away or selling copies.
I once dealt with large pdf documents, but back then everyone was using dial up and few could make CD-ROM copies on their home computer, so just having a very large pdf document was prevention enough. Few were willing to receive via email a copy that would take five hours to download, and fewer were willing to email it. The original was distributed on CD-ROM. You could, if you wanted, personalize the password and put the purchaser's name and address on the document itself.
If you want the ultimate is security, as a college student my dad had a part time summer job working in the computer section at some company (3M?), and he had to sign a form attesting that he was not a communist.