To Secure a PDF you will need the full blown version of Acrobat:

1. Open the file in Acrobat.

2. Click on File, Properties – this will open a dialog box with multiple tabs.

3. Click on the Security tab then click the down arrow by the field Security Method (No
    Security is listed by default).

4. Select Password Security and this will force the Password Security Settings Dialog Box to
    appear. This is where you will be making decisions about the document.

5. Where it says Printing Allowed it has None as the default so basically all you have to do is
    define a password. It is pretty neat how it works.