Hey Johnathan,
It depends on how you have setup your content programs. Is each piece of content have a program to manage it?
If so, I would use "Member of Smart Campaign" on the smart campaign that handles the content download.
If not, you could consider making a program for each piece of content and then each of your email campaigns could just reference the central program and you will have a way to report on total number of people who have downloaded it.
Let me know if you have any questions!
Hope this helps,
Jared