<local:MyAdvRichTextBox>
    <local:MyAdvRichTextBox.Document>
        <FlowDocument>
             <Paragraph>
                 (.....)
        </FlowDocument>
    </local:MyAdvRichTextBox.Document>
</local:MyAdvRichTextBox>

...
        public FlowDocument TextDocument
        {
            get { return (FlowDocument)GetValue(TextDocumentProperty); }
            set { SetValue(TextDocumentProperty, value); }
        }

        // Using a DependencyProperty as the backing store for TextDocument.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty TextDocumentProperty =
            DependencyProperty.Register("TextDocument", typeof(FlowDocument), typeof(UserControl1), new UIPropertyMetadata());
...

<loc:USRControl x:Class="AdvRichTextBox.UserControl1"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             xmlns:loc="clr-namespace:AdvRichTextBox"
             mc:Ignorable="d" 
             d:DesignHeight="300" d:DesignWidth="300" TextDocument="{Binding ElementName=txtMain, Path=Document, Mode=OneTime}">

...
        public FlowDocument TextDocument
        {
            get { return this.txtMain.Document; }
            set { this.txtMain.Document = value; }
        }
...