Hi ShellT,
It's a good question - unfortunately there's no features or options that would make doing what you have in mind easy or automatically behind the scenes. The basic methods are to offer a newsletter or incentives or run a contest - all intended for the visitor to fill out a form - you want them to provide their address willingly, else just snagging their address and spamming them later is not a good thing to do. Suggest setting up a form for your free PDF offer, if they want it, they will sign up for it. You could provide a preview of your PDF or explain why it should interest them.
