There is a work around to get Secured PDF on your reader. Though you may rightfully argue, not the best
Steps:
1. The Secured PDF must have printing enabled (most do).
2. You must have Adobe 7 (Later version crippled printing).
3. A PDF printer driver (Primo PDF is free and quite good)
4. Open the PDF an print it.
5. When you print. Depress the <Advanced> button.
6. Select the check box "Print as Image". (NOTE: The final PDF will be much larger because it is now an image for each page.)
7. Use a tool like PDFRead to convert the PDF to Mobi.
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