Download File with ajax

script language="javascript"

  // Get a PageRequestManager reference.
var prm = Sys.WebForms.PageRequestManager.getInstance();

// Hook the _initializeRequest event and add our own handler.
prm.add_initializeRequest(InitializeRequest);

function InitializeRequest(sender, args)
{
  // Check to be sure this async postback is actually 
  //   requesting the file download.
  if (sender._postBackSettings.sourceElement.id == "DownloadFile")
  {
    // Create an IFRAME.
    var iframe = document.createElement("iframe");

    // Get the desired region from the dropdown.
    var region = $get("Region").value;

    // Point the IFRAME to GenerateFile, with the
    //   desired region as a querystring argument.
    iframe.src = "GenerateFile.aspx?region=" + region;

    // This makes the IFRAME invisible to the user.
    iframe.style.display = "none";

    // Add the IFRAME to the page.  This will trigger
    //   a request to GenerateFile now.
    document.body.appendChild(iframe);
  }
}
/script

http://encosia.com/2007/02/23/ajax-file-downloads-and-iframes/