David,
The only way I've seen so far is to create the PDF using Distiller and then set an Open password in the Security settings. I can't think of another, easy, way to do this.
What are you using to create your PDFs? 'Well, it's one louder, isn't it?...
What we do is, if we need that extra push over the cliff, you know what we do?...Eleven. Exactly. One louder.'