test.cs

private bool IsValidZUGFeRD(InvoiceDescriptor invoice, Lines pdfInvoice) {
  // add key values to a list for validation
  List<string> validationValues = new List<string>();
  validationValues.Add(invoice.TaxTotalAmount.ToString(new CultureInfo("de-DE")));
  validationValues.Add(invoice.GrandTotalAmount.ToString(new CultureInfo("de-DE")));
  validationValues.Add(invoice.InvoiceNo);

  // check, if key values exist in visible PDF
  foreach (string value in validationValues) {
    if (pdfInvoice.Find(value).Count == 0)
      return false;
  }

  // all good
  return true;
}