In Acrobat Pro you can set a password on the PDF and allow users to only do certain things like filling in fields, or nothing at all.
I'm using Acrobat Pro 7 on Windows at work, and there's a "Secure" icon in the menu bar. Click on that, then click on "Show Security Settings", change the Security Method to "Password Security" then look under the "Permissions" section.
Not sure if there is a better way to do it but this is the way I've done it for a while and it works fine.