login without membership problem

I have a link on my first web page that redirects to another page which contains two dropdownlist
both dropdownlist containing some items
now when i select an item from dropdownlist
some links shows in view
and user click that link then a pdf file downloads
now my question is :
when a user is not logged in then it should redirects to login page
else able to download the file
how to do it
i am not using membership and my code is for this...

if(Session["UserName"]==null)
{
Response.Redirect("Redirect.aspx");
}