This functionality is not supported out-of-the-box. However, it could be implemented by the users as demonstrated in the Custom Code Formatting Language SDK example (https://github.com/telerik/xaml-sdk/tree/master/RichTextBox/CustomCodeFormattingLanguage).

When editing header/footer in MS Word, the paragraph which is inserted there has defined tab stops. In RadRichTextBox there isn't and thus, when the tab is pressed, a default tab stop is inserted.

System.Windows.Data Error: 2 : Cannot find governing FrameworkElement or FrameworkContentElement for target element. BindingExpression:(no path); DataItem=null; target element is 'SolidColorBrush' (HashCode=63604780); target property is 'Color' (type 'Color')

Line break has a specific property which should be taken into account only when there is a floating object which interrupts the line, e.g.:

<w:br w:type="textWrapping" w:clear="all" /> 

Showing a document that contains a Table object without any TableRow inside leads to NullReferenceException.

Workaround: After importing the document, remove the tables that don't have content:

foreach (var table in document.EnumerateChildrenOfType<Table>())
{
    if (table.Rows.Count < 1)
    {
        table.Parent.Children.Remove(table);
    }
}

The width of the table cell is calculated wrongly when it is set in the CSS style and through attribute: <td style="WIDTH: 75%" width=450>First cell</td>. The Unit isn't reset while reading the attribute value as its unit is missing and it is interpreted as % because the previous value is in percent.

Importing HTML document with white spaces and new lines separating the HTML elements will lead to additional intervals between words.

Currently floating images specified using the VML shapes are imported as non-floating, for example:

<v:shape id="_x0000_s1026" type="#_x0000_t75" style="height:180.25pt;margin-left:62.25pt;margin-top:1.65pt;position:absolute;width:180.25pt;z-index:251658240" o:preferrelative="t">
        <v:imagedata r:id="rId4" />
</v:shape>

When importing from a .docx the RichTextBox is adding a blank line for each continuous section break. You can see this in the attached file - in the RichTextBox there is an extra line between sections that does not exist in the source word document.

When a document has some section properties like orientation or page size and the document is printed with virtual printer, the generated PDF does not preserve the section properties. 

For example, if the document has Landscape orientation set, in the generated PDF file, the page with Landscape orientation is visualized with Portrait orientation and the text is drawn vertically.
If the document has some page size set, for example A5, in the generated PDF file, the page with size A5 is visualized as page with A4 page size.

After inserting page break or section break the visual position of the caret is wrong. It should be at the beginning of the paragraph instead of beginning of the page. It's observed for all type of breaks except a line break. 

When a table is dragged after a page break, and then Undo/Redo is performed for the drag operation NullReferenceException is thrown.

When a custom style is applied to a table and the document is exported to DOCX or RTF format, after importing and deleting the custom style, some properties like font size and foreground are preserved.

Currently, there is an SDK example (https://github.com/telerik/xaml-sdk/tree/master/RichTextBox/WindowsMetafileFormatImageDecoder) showing how to implement a custom decoder that can import WMF and EMF images. We could integrate this decoder in the source code and enable the users to import their documents that contain such type of images out-of-the-box. 

Available in R1 2017 Official Release Version.