Password Protection Enhancement and to Encrypt the Data within the PW Protected PDF. 3rd Party applications can remove PW Protection
Prevent 3rd Party applications to hack or remove password protected documents. Encrypt Password Protected PDF, so if it's hacked the sensitive data is not compromised.
Admincdas (Admin, Adobe) commented
Can you please name the third party app to remove the password so we can investigate further.
That can't be fixed, unfortunately. The password protection mechanism is described in the PDF ISO and Adobe follows it. Other software developers do not follow it, sadly. So do not rely on this encryption to protect the contents of your file.