Take link button in update panel.
Apply trigger to Link Button.
Write the Follwing Code in the link button click event.
System.Web.HttpResponse response = System.Web.HttpContext.Current.Response;
response.Clear();
response.AddHeader("Content-Disposition", "attachment; filename=" + "ABC.pdf" + ";");
response.ContentType = "application/excel";
response.TransmitFile("~//Downloadable//ABC.pdf");
response.End();
Note: You can change 'response.ContentType = "application/excel";' for excel document,also change file name that u want to download from server folder(eg.ABC.pdf to ABC.csv).