Restrict Content for only Contributors via .htaccess

You can create a custom page template for downloads and use something similar to the following:

if (current_user_can('contributor')) { 
    echo "<h2>Download Page</h2>"; 
} else { 
    echo 'You do not have access to this page";
}

You would need additional code to show the file you are offering for download.

Alternatively you could set another cookie and check value of that.