class SsrsTest
{
  public static void Test()
  {
    var url = string.Concat(
      "http://myserverurl/myreportname",
      "&rs:Command=Render&rs:Format=PDF&schema_id=1"
    );
    var request = WebRequest.Create(url);
    request.Credentials = CredentialCache.DefaultCredentials.GetCredential(new Uri("http://tempuri.org/"), "Basic");
    request.Timeout = 5 * 60 * 1000;
    request.Method = WebRequestMethods.Http.Get;
    try
    {
      using (var response = (HttpWebResponse)request.GetResponse())
      using (var responseStream = response.GetResponseStream())
      {
        if (responseStream == null)
          throw new ApplicationException("response is null");
        const int bufferSize = 1024;
        var buffer = new byte[bufferSize];
        var tmpFile = @"D:\Trash\456.pdf";
        if (File.Exists(tmpFile))
          File.Delete(tmpFile);
        using (var fs = new FileStream(tmpFile, FileMode.Create))
        using (var writer = new BinaryWriter(fs))
        {
          int count;
          do
          {
            count = responseStream.Read(buffer, 0, buffer.Length);
            if (count == 0) break;
            writer.Write(buffer, 0, count);
          } while (count > 0);
        }
      }
    }
    catch(WebException we)
    {
      var errorStream = we.Response.GetResponseStream();
      if (errorStream == null)
        throw;
      string s;
      using (var sr = new StreamReader(errorStream, Encoding.UTF8))
        s = sr.ReadToEnd();
      if (string.IsNullOrWhiteSpace(s))
        throw;
      Console.WriteLine(s);
      var serverErrors = Prepare500(s); // cos' web exception message here always is 'Internal server error (500)'
      if (string.IsNullOrEmpty(serverErrors))
        throw;
      throw new ApplicationException($"Ошибка получения отчета: {serverErrors}");
    }
    Console.WriteLine("done");
    Console.ReadKey(true);
  }

  static string Prepare500(string errorResponse)
  {
    try
    {
      var xd = new XmlDocument();
      using(var sr = new StringReader(errorResponse))
      using(var xr = XmlReader.Create(sr))
        xd.Load(xr);
      var errors = xd.SelectNodes("/html/body/ul//li/text()");
      if (errors == null)
        return null;
      var sb = new StringBuilder();
      foreach (XmlText xt in errors)
        if (!string.IsNullOrWhiteSpace(xt.Value))
          sb.AppendLine(xt.Value.Trim());
      return sb.Length == 0 ? null : sb.ToString();
    }
    catch(Exception e)
    {
      Console.WriteLine(e);
      return null;
    }
  }
}