.NET (Core and Framework)

<Window>
  <DockPanel LastChildFill="True">
    <StackPanel DockPanel.Dock="Bottom" Orientation="Horizontal">
      <Button Width="100" Height="25" Click="Save_Click">Save</Button>
    </StackPanel>
    <FlowDocumentScrollViewer DockPanel.Dock="Top" VerticalScrollBarVisibility="Auto">
      <FlowDocument x:Name="flowDoc" Background="White" PagePadding="5">
        <BlockUIContainer>
          <!--... Data bound UI stuff-->
        </BlockUIContainer>
      </FlowDocument>
    </FlowDocumentScrollViewer>
  </DockPanel>
</Window>

private void Save_Click(object sender, RoutedEventArgs e)
{
  SaveFileDialog dialog = new SaveFileDialog();
  dialog.DefaultExt = "xps";
  dialog.Filter = "XPS Document (*.xps)|*.xps";
  dialog.OverwritePrompt = true;

  if (dialog.ShowDialog() == true)
  {
    using (Package package = Package.Open(dialog.FileName, FileMode.Create, FileAccess.ReadWrite))
    {
      using (XpsDocument xpsDoc = new XpsDocument(package, CompressionOption.Maximum))
      {
        XpsSerializationManager rsm = new XpsSerializationManager(new XpsPackagingPolicy(xpsDoc), false);
        DocumentPaginator paginator = ((IDocumentPaginatorSource)flowDoc).DocumentPaginator;
        rsm.SaveAsXaml(paginator);
        rsm.Commit();
      }
    }
  }
}