The Portable Document Format, better known as PDF, is one of the most widely used document formats worldwide, and in order to ensure information confidentiality, this file format supports document encryption. In this paper, the authors analyze PDF encryption and show two novel techniques for breaking the confidentiality of encrypted documents.
You can download it from the following link: https://packetstormsecurity.com/files/download/154692/paper-pdf_encryption-ccs2019.pdf
Source: https://packetstormsecurity.com/files/154692/Practical-Decryption-exFiltration-Breaking-PDF-Encryption.html