Adobe ACROBAT 3D Manual page 27

If you want to create a secure document
Acrobat provides several methods of applying security:
Use digital signatures to indicate approval of a PDF document or form that you filled out.
(See Signing Adobe PDF documents.)
Certify documents to disallow subsequent changes. (See Certifying documents.)
Add passwords and set security options to restrict opening, editing, and printing PDF
documents. (See Adding passwords and setting security options.)
Encrypt a document so that only a specified set of users have access to it. (See Encrypting
Adobe PDF files using certificates.)
Apply server-based security policies to PDF documents. Server-based security policies are
especially useful if you want others to have access to PDF documents only for a limited
time. (See Encrypting Adobe PDF files using security policies.)
Apply the same security settings to a number of PDF documents by creating a user
security policy. (See Creating user security policies.)
Add security settings to PDF attachments, and use eEnvelopes. (See Using eEnvelopes to
send secure files.)