Hiding new words in a PDF document

A new algorithm based on PDF document structure and the new words generated from the hidden information is proposed in this paper. Through software Fontographer we generate two font files with the same character codes. The secrete key is stored in the first file and it contains the font shapes formed from the new words. The private blank characters are stored in the second file and they do not possess font shapes. The private blank characters are embedded in a PDF document, and the secrete key are transmitted to the third party. In the watermark extraction stage, we need download the secrete key from the third party, and then replace the private blank characters´ font with it. Since the embedded watermark is blank characters and is overlapped with the original page objects in the PDF document, it will not affect the content, as well as the format of the PDF documents. These guarantee the security and the robustness of the watermark. In a word, our algorithm is simple, robust, and secure.