The article states that there is a new threat which hackers can unauthorized access and change to encrypted PDF without knowing password. This threat is called PDFex attacks which hackers remotely use the technique to remotely exploit PDF data. This means using this attack can automatically send the decrypted file out by using a remote-controlled server. Vulnerabilities were found in various well-known software such as Adobe Acrobat, Foxit Reader, and Nitro Reader and multiple browsers such as Chrome, Firefox, and Safari. Additionally, a team of German security researchers found 2 weaknesses of PDF encryption which are partial encryption and ciphertext malleability.
Link : https://thehackernews.com/2019/10/pdf-password-encryption-hacking.html