[Authorize]
public async Task<IActionResult> Index(int clientId, string error)
    {
     // ....... Removed
     var model = new SecretIndexModel()
        {
            Client = clients.Client,
            ErrorMessage = error
        };
        return View(model);
    }

[Authorize]
[HttpPost]
public async Task<IActionResult> Add(SecretIndexModel model)
{

 /// removed ....

 return DownloadDocument("results json string");
 }

public FileResult DownloadDocument(string id)
    {
        var ms = new MemoryStream(System.Text.Encoding.ASCII.GetBytes("abc"));

        return File(ms, System.Net.Mime.MediaTypeNames.Application.Octet, "secret.json");
    }

return RedirectToAction("Index",
       new
          {
           clientId = model.Client.Id,
           error = (client.Errors == null)
                   ? string.Empty
                   : client?.Errors?.FirstOrDefault()?.Message
          });