1. You Can Simpally Use window.open(fileurl); command in javascript. it will directly open a file if browser support other wise shows download dialog box.
or you can use a proper download button i.e
string sFilename;
sFilename = Server.MapPath("todown.doc");
byte[] btFile = System.IO.File.ReadAllBytes(sFilename);
Response.Clear();
Response.AddHeader("Content-Disposition", "attachment;filename=" + sFilename + "");
Response.OutputStream.Write(btFile, 0, btFile.Length);
Response.End();