Protecting an eBook on CD

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The main problem looks the price for protection.
For example you can use SHK key to protect the acrobat reader and the only one who have the right key with protected acrobat can read it. This will increase the cost of the book about 30-50$ which looks like unacceptable if the price of ebook is only 5$.
I think you cant find ready made strong protection which will not increase too much the price of the ebook. The way is make some own custom thing.

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