A
Anonymous
Guest
That's complicated 
If you already have a login / password implementation working in your site, you just have to filter the link that take users to the download action.
You can do it sending the users when they click the link to a page that searches for the "variable session" (the variable that the user needs to be logged in) and then, if found pop-up the download.
The only matter is that many people is using pop-up filters, so the download pop-up won't appear for them...
You can warn it besides the link.
If you already have a login / password implementation working in your site, you just have to filter the link that take users to the download action.
You can do it sending the users when they click the link to a page that searches for the "variable session" (the variable that the user needs to be logged in) and then, if found pop-up the download.
The only matter is that many people is using pop-up filters, so the download pop-up won't appear for them...
You can warn it besides the link.