To reproduce save the following content in a .xaml fail and load it in RadRichTextBox:
<t:RadDocument xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:t="clr-namespace:Telerik.Windows.Documents.Model;assembly=Telerik.Windows.Documents" xmlns:s="clr-namespace:Telerik.Windows.Documents.Model.Styles;assembly=Telerik.Windows.Documents" version="1.2" LayoutMode="Flow" LineSpacing="1.15" LineSpacingType="Auto" ParagraphDefaultSpacingAfter="0" ParagraphDefaultSpacingBefore="0" SectionDefaultPageMargin="6,6,6,6" SectionDefaultPageSize="816,1056" StyleName="defaultDocumentStyle">
  <t:RadDocument.ProtectionSettings>
    <t:DocumentProtectionSettings EnableDocumentProtection="False" Enforce="False" HashingAlgorithm="None" HashingSpinCount="0" ProtectionMode="ReadOnly" />
  </t:RadDocument.ProtectionSettings>
  <t:RadDocument.Styles>
    <s:StyleDefinition DisplayName="Document Default Style" IsCustom="False" IsDefault="False" IsPrimary="True" Name="defaultDocumentStyle" Type="Default">
      <s:StyleDefinition.ParagraphStyle>
        <s:ParagraphProperties LineSpacing="1.15" SpacingAfter="0" />
      </s:StyleDefinition.ParagraphStyle>
      <s:StyleDefinition.SpanStyle>
        <s:SpanProperties FontFamily="Verdana" FontSize="16" FontStyle="Normal" FontWeight="Normal" />
      </s:StyleDefinition.SpanStyle>
    </s:StyleDefinition>
    <s:StyleDefinition DisplayName="TableNormal" IsCustom="False" IsDefault="True" IsPrimary="True" Name="TableNormal" Type="Table" />
  </t:RadDocument.Styles>
  <t:Section>
    <t:Paragraph>
      <t:Span Text="Hello World" />
    </t:Paragraph>
  </t:Section>
</t:RadDocument>