Download file after fill in email address
The easiest way without resorting to coding would be setting up a Webform that the user is required to fill out, containing only one email field, and putting the link to the PDF in the completion message.

If you need strict access control on the PDF, you can setup your site with Email Registration and Generate Password, and redirect the user after registration to a node that is visible only to authenticated users. Add a file field using the private file system to the content type and attach the PDF to the node.