@diablokicks:
It appears we're already working with you on this matter via email, so we can continue the discussion there if necessary.
Generally speaking, any PDF which is under 20 MB and not password-locked should work fine, while password-locked PDF files must have their security compatibility set to "Acrobat 5.0 or higher".
If you are testing the PDF Stamping with your file, make sure you wait at least 15 minutes after uploading the file before you try performing a test checkout. Also note that PDF Stamping will not work with the "Send free download link" function in your Seller Admin.