Best way to make PDF's available, and track downloads

"Ideally I'd like to have a popup when a user is about to leave a page to encourage engagement."

Personally, I find these very annoying.  But do a Google search for Exit Popups.  You'll find plenty of examples online.

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Nothing is easy anymore.  It depends a lot on what kind of data you want to gather and which regions of the world your users reside in as there are UK, EU and GDPR privacy laws  to consider.  Your privacy policy and cookie statements will need to be transparent about what you're doing with collected data.  Also you must provide users with the means to opt out if they desire it.  So you should be discussing this with your stake holders, web developers & legal department.

That is a function of the HTML in your web page

Please post the name of the Adobe program you use to create your web page so a Moderator may move this message to that forum