I've created an information pdf and I want to gather email addresses in exchange for downloading the pdf file. How do I do that?

I would suggest you to add an email autoresponder to your form, the autoresponder email will allow you to attach a file on it, this way the users receive the email with the attached file as soon as they submit the form and the information to you. 

Guide: https://www.jotform.com/help/26-Setting-up-an-Autoresponder-Email 

I hope this helps.