Need help building a E-book library for newsletter subcribers/members only

I want to build a E-Book library, the E-books (PDF's) must only be available to read or download when a visitor registered to my Mailchimp newsletter and/or is registered on my site.

When a new visitor comes on the site , a popup should be shown that asks the visitor to register to the newsletter in order to gain acces to the E-Book Library downloads.

I'm not quite sure how I can accomplish this and what WPMU DEV plugin(s) I can best use for this.

So in short I want:
- protect my e-book (PDF) downloads so that only newsletter subscribers can obtain them
-show a popup to (new) visitors asking them to to register to my mailchimp newsletter in order to obtain download acces.

I hope you can help me achieving this !

Regards,

Michel Kok