using System.IO;
    using System.IO.Packaging;
    using System.Windows;
    using System.Windows.Documents;
    using System.Windows.Xps.Packaging;
    using System.Windows.Xps.Serialization;
    namespace WpfApp3
    {
        public partial class MainWindow : Window
        {
            public MainWindow()
            {
                InitializeComponent();
            }
            public static void SaveAsXps(string path, FlowDocument document)
            {
                using (var package = Package.Open(path, FileMode.Create))
                {
                    using (var xpsDocument = new XpsDocument(package, System.IO.Packaging.CompressionOption.Maximum))
                    {
                        var xpsSerializationManager = new XpsSerializationManager(new XpsPackagingPolicy(xpsDocument), false);
                        var documentPaginator = ((IDocumentPaginatorSource)document).DocumentPaginator;
                        xpsSerializationManager.SaveAsXaml(documentPaginator);
                    }
                }
            }
    
            private void Button_Click(object sender, RoutedEventArgs e)
            {
                { 
                    string sFileName = @"C:\...\printPreview.xps";
    
                    XpsDocument doc = new XpsDocument(sFileName, System.IO.FileAccess.Read);
                    viewer.Document = doc.GetFixedDocumentSequence();
                    return;
                }
            }
    
            private void Button_Click_1(object sender, RoutedEventArgs e)
            {
                SaveAsXps(@"C:\...\printPreview.xps", flowDocument);
            }
        }
    }