download file from url with ajax and asp.net mvc 5

I am trying to download a file from a url with ajax and asp.net mvc 5 this is my controller:

[HttpPost]
public ActionResult DownloadDocument(DownloadModel model){
   urlFile = "http://www.storage.net/data/pdf-xml/" + model.nombre.ToLower() + model.Extencion;
   //Create a stream for the file
   Stream stream = null;
   //This controls how many bytes to read at a time and send to the client
   int bytesToRead = 10000;
   // Buffer to read bytes in chunk size specified above
   byte[] buffers = new Byte[bytesToRead];
   // The number of bytes read
   try{
     //Create a WebRequest to get the file
     HttpWebRequest fileReq = (HttpWebRequest)HttpWebRequest.Create(urlFile);
     //Create a response for this request
     HttpWebResponse fileResp = (HttpWebResponse)fileReq.GetResponse();
     if (fileResp.ContentLength > 0)//aquí reviso si el archivo contiene datos (existe o no)
     {
       if (fileReq.ContentLength > 0)
         fileResp.ContentLength = fileReq.ContentLength;
       //Get the Stream returned from the response
       stream = fileResp.GetResponseStream();
       // prepare the response to the client. resp is the client Response
       //var resp = HttpContext.Current.Response;
       //Indicate the type of data being sent
       Response.ContentType = "application/octet-stream";
       //Name the file 
       Response.AddHeader("Content-Disposition", "attachment; filename=\"" + model.nombre + model.Extencion + "\"");
       Response.AddHeader("Content-Length", fileResp.ContentLength.ToString());
       int length;
       do{
         // Verify that the client is connected.
         if (Response.IsClientConnected){
           // Read data into the buffer.
           length = stream.Read(buffers, 0, bytesToRead);
           // and write it out to the response's output stream
           Response.OutputStream.Write(buffers, 0, length);
           // Flush the data
           Response.Flush();
           //Clear the buffer
           buffers = new Byte[bytesToRead];
         }else{
           // cancel the download if client has disconnected
           length = -1;
         }
       } while (length > 0); //Repeat until no data is read
     }
   }catch (Exception e){
     d2.Add("Error", e.Message);
   }finally{
     if (stream != null){
       //Close the input stream
       stream.Close();
     }else if (d2.Count < 1){
       d2.Add("Error", "El Archivo no existe");
     }
   }
   //mostramso el mensaje de que no existe el archivo
   return Json(serializer.Serialize(d2));
 }  

If the form is directed to the controller, it performs the download, but if a file did not exist, it would send me a message which I show in my view but it is shown in a separate view.
And I want the download to be done through jquery ajax since this is the way I can show the message, but the download does not appear in the browser
This is my form:

<form method="post" class="form-horizontal" role="form" name="formDownload" id="formDownload140464">
  <input type="hidden" value="AAA010101AAA" id="rfcCompany" name="rfcCompany">
  <input type="hidden" value="40464" id="id" name="id">
  <input type="hidden" value=".pdf" id="Extencion" name="Extencion">
  <input id="Nombre" name="Nombre">
  <button type="button" class="btn btn-primary btn-sm previewPopover" title="Descargar" onclick="MostrarMensaje(40464)">
    <span class="elusive icon-download-alt"></span>
  </button>
</form> 

My ajax function is this, I use it to display the message.

function MostrarMensaje(id) {
    $('#formDownload1' + id).on("submit", function (e) {
        e.preventDefault();
        $.ajax({
            type: 'POST',
            url: '/Portal/DownloadDocument',
            data: $('#formDownload1' + id).serialize(),
            success: function (data) {
                var d = JSON.parse(data);
                $.jGrowl("" + d["Error"], {
                    header: "Error",
                    //sticky: true,
                    theme: "red"
                });
            }
        });
    });
} 

With this the message is displayed but the download does not perform.
I have seen that many pass the url in the data and download with window.location.href = d["Url"];, but my problem here with this code is that I will not know if the file exists or not and it will download an empty file.