private void SignatureDefinitionCommandHandler(object sender, RoutedEventArgs e)
{
    SignatureDefinition sigDefDialog = new SignatureDefinition();
    if (sigDefDialog.ShowDialog() == System.Windows.Forms.DialogResult.OK)
    {
        XpsSignatureDefinition signatureDefinition = new XpsSignatureDefinition();
        signatureDefinition.RequestedSigner = sigDefDialog.RequestedSigner.Text;
        signatureDefinition.Intent = sigDefDialog.Intent.Text;
        signatureDefinition.SigningLocale = sigDefDialog.SigningLocale.Text;
        try
        {
            signatureDefinition.SignBy = DateTime.Parse(sigDefDialog.SignBy.Text);
        }
        catch (FormatException)
        {
        }
        signatureDefinition.SpotId = Guid.NewGuid();
        IXpsFixedDocumentSequenceReader docSeq = _xpsDocument.FixedDocumentSequenceReader; //_xpsDocument is type System.Windows.Xps.Packaging.XpsDocument
        IXpsFixedDocumentReader doc = docSeq.FixedDocuments[0];
        doc.AddSignatureDefinition(signatureDefinition);
        doc.CommitSignatureDefinition();
        InitializeSignatureDisplay();
    }
}

ShowStatus("   Getting FixedDocumentSequenceReader.");
IXpsFixedDocumentSequenceReader fixedDocSeqReader =
    xpsDocument.FixedDocumentSequenceReader;

ShowStatus("   Getting FixedDocumentReaders.");
ICollection<IXpsFixedDocumentReader> fixedDocuments =
    fixedDocSeqReader.FixedDocuments;

ShowStatus("   Getting FixedPageReaders.");
IEnumerator<IXpsFixedDocumentReader> enumerator =
    fixedDocuments.GetEnumerator();
enumerator.MoveNext();
ICollection<IXpsFixedPageReader> fixedPages =
    enumerator.Current.FixedPages;

// Add a document structure to each fixed page.
int i = 0;
foreach (IXpsFixedPageReader fixedPageReader in fixedPages)
{
    XpsResource pageStructure;
    ShowStatus("   Adding page structure resource:\n       '" +
                       Filename(_fixedPageStructures[i]) + "'");
    try
    {   // Add a new StoryFragment to hold the page structure.
        pageStructure = fixedPageReader.AddStoryFragment();
    }
    catch (System.InvalidOperationException)
    {
        MessageBox.Show(xpsUnstructuredFile +
            "\n\nDocument structure cannot be added.\n\n" +
            Filename(xpsUnstructuredFile) + " might already " +
            "contain an existing document structure.",
            "Cannot Add Document Structure",
            MessageBoxButton.OK, MessageBoxImage.Error);
        break;
    }

    // Copy the page structure to the new StoryFragment.
    WriteResource(pageStructure, _fixedPageStructures[i++]);
}

ShowStatus("   Saving and closing the new document.\n");
xpsDocument.Close();