To display this dialog box, select Security > Security > Manage Security > Modify, then choose Password Security in the Security Method drop-down list.
Specify compatibility level (40-bit, 128-bit for different PDF versions, 128-bit or 256-bit AES - Advanced Encryption Standard)
Choose from the options: Encrypt all document contents, All contents except metadata, or File attachments only (options depend on the encryption level)
Secure the document by an Open or a Permissions type password or both:
Open Password: Set this as a prerequisite for opening the current PDF document in future.
Permissions Password: Set this and choose permitted actions from the drop-down lists for printing and changing the document, enable or disable copying different contents, enable or disable accessibility for screen readers to read aloud textual contents for visually impaired. This password will be needed to modify these settings or to change passwords in future.
Click OK to finish and confirm the Open and/or Permissions Password in the pop-up Password dialog box.