Let's say you have a link to your download page

<a class=downloadLink href=www.example.com/foo.pdf>Download</a>

In your Javascript:

$(.downloadLink).click(

    function(e) {   

        e.preventDefault();



        //open download link in new page

        window.open( $(this).attr(href) );



        //redirect current page to success page

        window.location=www.example.com/success.html;

        window.focus();

    }

);