On the steganographic image based approach to PDF files protection
This addresses the issue of image protection in PDF files for users needing secure document handling, but it is incremental as it compares existing watermarking techniques.
The paper tackled the problem of protecting digital images within PDF documents from unauthorized copying by proposing two watermarking schemes that extract images, embed a binary pattern using frequency or spatial domain techniques, and return them to the PDF. It found that spatial domain LSB embedding is more preferable than frequency domain DWT for both visible and invisible watermarking.
Digital images can be copied without authorization and have to be protected. Two schemes for watermarking images in PDF document were considered. Both schemes include a converter to extract images from PDF pages and return the protected images back. Frequency and spatial domain embedding were used for hiding a message presented by a binary pattern. We considered visible and invisible watermarking and found that spatial domain LSB technique can be more preferable than frequency embedding using DWT.