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You could take the Hollywood screener approach and every few pages inject some customized text like " FOR THE PERSONAL USE OF JOHN SMITH ONLY. NOT FOR DISTRIBUTION UNDER PENALTY OF LAW. THIS DOCUMENT CONTAINS HIDDEN SECURITY TRACKING FEATURES."
The last part doesn't even have to be true, it's just to discourage more casual sharing.
epub and mobi simply do not have security features of the sort available for PDF, and even secured PDFs can be improperly handled and copied (just ask the TSA....).
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