How can I know what downloads the user do?

I mean, If I have the follow products
Product1
Product2
Product3
Product4
Product5
and the user1, user2
I would determine what products the user download
Example:
user1 product1, product3
user2 product1, product5
I have to determine the product downloads for the users to use that information in our newsletters.
By the way, I will change the registration form and I will ask to the user about receive newsletter from us.
Best regards,
Teksure