This happens when documents are saved or sent in the "Secure PDF Single-Page" or "Secure PDF Multi-Page" format using any of the features listed below, and then they are opened in Adobe Acrobat Reader's version 23.003.20201 or later versions:

• Scan to USB
• Scan to FTP(SFTP)
• Scan to Network
• Scan to Email Server
• Scan to SharePoint
• Scan to HTTP(S)
   

Try these recommended solutions:

• Click here and download the SecurePDF Repair Tool.  Start the downloaded tool and follow the on-screen instructions.
• Right-click on the PDF document you want to open and select to open it with Google Chrome or Microsoft Edge.

Update your Brother machine's firmware to the latest version to completely solve this issue.