Add a document password

With the PDF document open, choose Document > Secure > Show Security Settings For This Document.

Choose Password Security from the Security Method menu.

From the Compatibility menu, choose the earliest version of Acrobat that you want to be able to open this document.

Select Require A Password To Open The Document.

In the Document Open Password box, type the password (case-sensitive).

Click OK. Type the Document Open password again, and then click OK.