Its very easy. Use a session instead of direct download.
Like here is the ppt location
www.mysite.com/file1.ppt
So now only registerd user must access. So when the user logs in set a session like $_SESSION['download'] = 1
then dont allow the user directly download the file.ppt
Create a DB with the file location and
create a file called download.php
and in that download.php create a if loop like
if( $_SESSION['download'] == 1){
here query the DB with download location and pull the file
}
else
{
You have no permission to download this file
}
Thats all. Hope it helps you