How to modify a pdf file so its text cannot be exported.

.
How can one modify a pdf file that was created from a postscript
source with 'ps2pdf' so that it can be offered as a compressed
"ebook" but with its contents protected so the text cannot be
de-compiled or exported (as with 'pdftotext' or something similar)?

Also, any suggestions on the best way to offer an 'ebook' for
immediate download on the 'net (either with or without charging
a fee for it)?

I have had some requests for a couple of books I have published
to be made available as ebooks.


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