Bold text in Flow document

<FlowDocumentPageViewer xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <FlowDocument ColumnWidth="700">
   

    <Paragraph><Rectangle Fill="Red" Height="1" Width="400" HorizontalAlignment="Left" /><LineBreak/></Paragraph>
    
    <Paragraph><Bold>XAMLPad</Bold> </Paragraph >
   
    </FlowDocument>
</FlowDocumentPageViewer>