Email before download

Not something that I have done, but your best approach is some careful searching of the Plugin Directory, and possibly the Theme Directory.

The only plugin that looked promising in my quick search was:
https://www.ads-software.com/plugins/sell-media/

You would have to see if it allows you to set a price of zero and not force payment.

Much to my surprise, this search is really promising:
https://www.ads-software.com/plugins/search.php?q=email+before+download

Among others, it turned up this popular plugin:
https://www.ads-software.com/plugins/email-before-download/

I looked at email before download, looks like what I need but its only compatible to 4.0.12. I’m using 4.6

Although the entry says that, this post leads me to believe that it worked properly as of 4.5.3:
https://www.ads-software.com/support/topic/issues-with-latest-wp-update?replies=2

I think it is worth trying, especially given the 20,000+ active installations using it.