You shouldn't do this in your ASP.NET code, because that runs on the server. This case is better handled client-side, using JavaScript. JavaScript has a
setTimeout function[
^] which you can use to run a function after a certain amount of time. In this function, you can redirect the user to a PDF file.
setTimeout(function() {
window.location = "/path/to/your/file.pdf";
}, 5000);