How to create a lead magnet on shopify through a third party theme

[Dominika96]

I have set up a pop up with the intention to generate leads for the business. The pop up offers a free e book which the potential customer would have to enter their email in order to obtain it. 

I have set up a CDN link so the customers can download the pdf but there is no way of downloading the ebook I have produced. 

How could this be fixed ASAP?

I want my customers to fill out the form in the pop up (so I have their email on shopify) and then once that process is done, they are taken to a page where they can download the PDF. 

I have looked at the documentation and it did not help. Additionally, I have got in contact with Shopify and this is what they have said:

"So, it may be possible to do this within your theme file, but the theme developers would be best placed to advise on where you can redirect a link to, once a customer completes a popup contact form, the landing page is set within the theme. There are some page builder apps which can be used to create new pages but with regard to how the popup form redirects to these, that would be managed within the theme file also"

How could I create a way for the lead magnet to be downloaded after a potential customer signs up with their email?? I don't mind whether there's something within the pop up itself, or they are redirected to a landing page.

Any help would be appreciated.

Thanks