How can I save content from richtextbox into a RTF file?

FlowDocument document = rtt.Document;
System.IO.Stream ms = new System.IO.MemoryStream();
System.Windows.Markup.XamlWriter.Save(document, ms);
byte[] data = new byte[ms.Length];
ms.Position = 0;
ms.Read(data, 0, data.Length);
ms.Close();
File.WriteAllBytes(THE PATH, data);

By the code above, I put the content of a richtextbox（named as rtt）into a stream, then save this stream into a .rtf file. 

Since I nested a small richtextbox into the big richtexbox(named as rtt) by BlockUIContainer, so I think I can not use  richTextBox.Document.ContentStart,
 richTextBox.Document.ContentEnd, (i suppose this is only suitable for pure text?) I tried to convert the content in richtextbox into binary stream.

But the problem is, in the saved file, the content from the richtextbox is shown in below format:

<FlowDocument PagePadding="5,0,5,0" AllowDrop="True" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"><Paragraph>RichTextBoxggg<Run xml:lang="en" xml:space="preserve"> it is just a test!!!!</Run></Paragraph><BlockUIContainer><RichTextBox IsReadOnly="True" Name="rtb2"><FlowDocument PagePadding="5,0,5,0" AllowDrop="True"><Paragraph>sdlf</Paragraph></FlowDocument></RichTextBox></BlockUIContainer></FlowDocument>

It is shown as xaml format, but i want it shown as the format in WPF (the view when i design  in WPF, ), 

So How can I convert the content of a richtexbox to binary stream, and then save it as a rtf file in a disk (and show as the wpf view, not the xaml format)