You can add security to any document by embedding it in an encrypted envelope, called a security envelope (or eEnvelope, in earlier versions) and sending it as an email attachment. This method is especially useful if you want to send a secure file attachment without modifying the attached file. When other users open the security envelope, they can extract the file attachments and save them to disk. The saved files are identical to the original file attachments and are no longer encrypted when saved.
For example, suppose that you want to send several documents, including non-PDF documents, to your accountant, but you don’t want anyone else to view the documents. You can embed these documents as file attachments in a security envelope, encrypt the security envelope so that only your accountant can open the attachments, and then email the envelope. Anyone can open the envelope, view its cover page, and even view a list of the contents of that envelope, but only your accountant can view the embedded attachments and extract them to the computer.
When you create a secure attachment, you’re prompted to select or create a security policy.

in
the Tasks toolbar, and choose Create Security Envelope.