Protect a PDF to sell as a download

I have designed a website for an author who wants to sell his book as a physical book and also as a download. The book has over 200 images and a lot of custom formatting and is proving to be a nightmare to convert to an ebook. I've seen many authors selling their books as PDF downloads. What is the best way to protect the PDF file? The book is 396 pages long. The PDF is about 85 MB, is that too large to offer as a download? Are there ways to make the file smaller (other than zip)?
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Karl Heinz KremerCommented:
The best way is to add something to every copy that the buyer is not willing to share. You could stamp each page with the buyer's credit card number, and then protect the file with the standard PDF document security (don't allow modifications or content extraction). This way, the buyer very likely will not want to share that file with anybody. If you don't have access to the credit card number, use the customer's name and email address ("Copy prepared for John Doe (") and stamp every page with that.

I've bought a few books online that use that mechanism.

This does require that your web server allows to run a custom program to generate a download. I would use e.g. iText or iTextSharp or to modify the PDF document just before the download.

85MB is a large PDF file. Zipping that file will very likely not make the file smaller, you may even end up with a larger file because PDF files are already compressed (that is, if you've used the right tools to create them). I would use Adobe Acrobat Pro and analyze the file to see where all that space is used. Depending on what you want user's to do with that file, it may not be necessary to have high resolution images in the document, and you can downsample these images to a lower resolution. Again, Adobe Acrobat pro can do that for you, and you have full control over how a file gets processed and shrunk.

Let me know if you need more information about either aspect of this answer.

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You might want to contact Andy Hunt at the Pragmatic Bookshelf

Andy does real-time PDF publishing, where he imbeds buyer-identifying data in the PDF before sending it.  If a buyer posts the PDF, then he/she becomes liable for copyright violation.

He probably does some other protective stuff.

Your client might consider using a third-party, like PragProg, do their online publishing and distribution.
nociSoftware EngineerCommented:
As mentioned before stamping identifiable info on each page should work. But a credit card # seems unwise though, if someone read a document over someelses shoulder you do publish private stuff as publisher seems inapproprate too.
JustDuckyDesignsAuthor Commented:
Excellent advice. Extremely helpful. Thank you!
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