The following code creates a RichTextBox in WPF and adds a flow document to it.
The RichTextBox element in XAML represents WPF RichTextBox control.
<Window x:Class="RichTextBoxSample.Window1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="Window1" Height="300" Width="300">
<Grid>
<RichTextBox Name="RTB"
Height="167" Margin="0,0,0,95"
Width="278">
<FlowDocument>
<Paragraph>
<Run>First paragraph goes
here. Type some text here.</Run>
</Paragraph>
<Paragraph>
<Run>Second paragraph goes
here. Type some text here.</Run>
</Paragraph>
</FlowDocument>
</RichTextBox>
</Grid>
</Window>