Blank Pdf file getting downloaded

Hi

  I am using NimbusPost manifest APi. It is getting blank file downloaded

public void Manifest(string awb, string token)
{
    var client = new RestClient("htps://api.nimbuspost.com/v1/shipments/manifest");
    client.Timeout = -1;
    var request = new RestRequest(Method.POST);
    request.AddHeader("Content-Type", "application/json");

    string awbNo = awb;
    string val1 = "NMBC0001789312";
    var body = $@"{{
        ""awbs"": [
            ""{awbNo}"",
            ""{val1}""
        ]
    }}";

    request.AddParameter("application/json", body, ParameterType.RequestBody);
    request.AddHeader("Authorization", "Bearer " + token);
    IRestResponse response = client.Execute(request);

    dynamic json = JsonConvert.DeserializeObject(response.Content);
    string pdf = json.data;

    string s = pdf;
    int idx = s.LastIndexOf('/');
    string fileName = s.Substring(idx + 1);
    using (var client5 = new System.Net.WebClient())
    {
        byte[] bytes5 = client5.DownloadData(pdf);
        File.WriteAllBytes(Server.MapPath("~/App_Data/") + fileName, bytes5);
    }
}

Thanks