The RichTextBox internal logic that imports .docx documents (DocxFormatProvider) doesn't support elements that don't need direct content, but have both start and end tags. Instead such elements should use self-closing tags. 

For example, instead of <w:vertAlign w:val="baseline"></w:vertAlign> it should be <w:vertAlign w:val="baseline"/>

When such element is read during import, issues may occur. Two main known issues here are with the w:vertAlign and Relationship tags.

Using

<w:vertAlign w:val="baseline"></w:vertAlign> 

instead of

<w:vertAlign w:val="baseline"/> 

throws NullReferenceException with the following stack trace:

System.NullReferenceException: 'Object reference not set to an instance of an object.'
Telerik.Windows.Documents.FormatProviders.OpenXml.dll!Telerik.Windows.Documents.FormatProviders.OpenXml.OpenXmlHelper.ConvertStringToBaselineAlignment(string value) Line 168 C#  
Telerik.Windows.Documents.FormatProviders.OpenXml.dll!Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Import.SpanStyleImporter.ReadBaselineAlignment(Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Parsing.Style style) Line 243 C#  
Telerik.Windows.Documents.FormatProviders.OpenXml.dll!Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Import.SpanStyleImporter.ReadStyle(Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Parsing.Style style) Line 88 C#  
Telerik.Windows.Documents.FormatProviders.OpenXml.dll!Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Import.StyleDefinitionsImporter.ReadSpanStyleProperties(Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Parsing.Style style) Line 278 C#  
Telerik.Windows.Documents.FormatProviders.OpenXml.dll!Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Import.StyleDefinitionsImporter.ReadStyle() Line 188 C#  
Telerik.Windows.Documents.FormatProviders.OpenXml.dll!Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Import.StyleDefinitionsImporter.Import() Line 67 C#  
Telerik.Windows.Documents.FormatProviders.OpenXml.dll!Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Import.DocxImporter.ReadXmlContentFromPackage(Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Import.DocxPartImporterBase importer) Line 430 C#  
Telerik.Windows.Documents.FormatProviders.OpenXml.dll!Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Import.DocxImporter.ReadXmlContentAndRelationsFromPackage(Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Import.DocxPartImporterBase importer) Line 406 C#  
Telerik.Windows.Documents.FormatProviders.OpenXml.dll!Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Import.DocxImporter.Import() Line 190 C#  
Telerik.Windows.Documents.FormatProviders.OpenXml.dll!Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.DocxFormatProvider.Import(System.IO.Stream input) Line 147 C#
Telerik.Windows.Documents.dll!Telerik.Windows.Documents.FormatProviders.DocumentFormatProviderBase.Import(byte[] input) Line 90 C#

Using

<Relationship Id="rId00004" Target="styles.xml" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/styles"></Relationship> 

instead of

<Relationship Id="rId00004" Target="styles.xml" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/styles"/> 

throws KeyNotFoundException with the following stack trace:

KeyNotFoundException: The given key was not present in the dictionary.
    System.ThrowHelper.ThrowKeyNotFoundException() in ThrowHelper.cs
    System.Collections.Generic.Dictionary<TKey, TValue>.this[TKey].get(TKey) in Dictionary.cs
    Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Import.PartRelationsImporter.GetRelationByID(string) in PartRelationsImporter.cs
    Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Import.DocxImporter.GetRelationByID(string) in DocxImporter.cs
    Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Import.MainDocumentImporter.BuildSection() in MainDocumentImporter.cs
    Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Import.MainDocumentImporter.BuildBody() in MainDocumentImporter.cs
    Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Import.MainDocumentImporter.BuildDocument() in MainDocumentImporter.cs
    Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Import.MainDocumentImporter.Import() in MainDocumentImporter.cs
    Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Import.DocxImporter.ReadXmlContentFromPackage(Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Import.DocxPartImporterBase) in DocxImporter.cs
    Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Import.DocxImporter.ReadXmlContentAndRelationsFromPackage(Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Import.DocxPartImporterBase) in DocxImporter.cs

To work this around, import the document with the WordsProcessing library. If you need to display it in RadRichTextBox, you can export it again using the WordsProcessing library. This will normalize the tags. Then import it again using the RadDocument's DocxFormatProvider.

 var dplDocxFormatProvider = new Telerik.Windows.Documents.Flow.FormatProviders.Docx.DocxFormatProvider();
 RadFlowDocument flowDocument = dplDocxFormatProvider.Import(File.ReadAllBytes("../../../test.docx"), null);

 var docBytes = dplDocxFormatProvider.Export(flowDocument, null);
 File.WriteAllBytes("../../../modified-doc.docx", docBytes);

 var rtbDocxFormatProvider= new Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.DocxFormatProvider();
 RadDocument document = rtbDocxFormatProvider.Import(File.ReadAllBytes("../../../modified-doc.docx"));

Currently, the only customization that can be performed is using HyperlinkToolTipFormatString property of RadRichTextBox, which sets the format for all hyperlinks in the document of the control. It would be nice if it were possible to be able to set different tooltips. 

Bold/Italic does not work for some font styles.

This might be a framework limitation in WPF: https://www.telerik.com/forums/some-fonts-are-not-able-to-bold-italic

Sample fonts list:

1. Tw Cen MT Condensed Extra Bold (bold/italic)
   
2. Tw Cen MT Condensed (bold/italic)
   
3. Sitka Text Semibold (italic)
   
4. Sitka Subheading Semibold (italic)
   
5. Sitka Small Semibold (italic)
   
6. Sitka Heading Semibold (italic)
   
7. Sitka Display Semibold (italic)
   
8. Sitka Banner Semibold (italic)
   
9. Segoe UI Variable Text Semibold (italic)
   
10. Segoe UI Variable Text Light (bold/italic)
    
11. Segoe UI Variable Display Light (bold/italic)
    
12. Segoe UI Semilight (bold/italic)
    
13. Segoe UI Semibold (bold/italic)
    
14. Segoe UI Light (bold/italic)
    
15. Segoe UI Black (italic)
    
16. Script MT Bold (bold/italic)
    
17. Rockwell Condensed (bold/italic)
    
18. Rockwell Extra Bold (italic)
    
19. Rage Italic (bold/italic)
    
20. OCR A Extended (bold/italic)
    
21. Nirmala UI Semilight (bold/italic)
    
22. Microsoft YaHei UI Light (bold/italic)
    
23. Microsoft YaHei Light (bold/italic)
    
24. Microsoft JhengHei UI Light (bold/italic)
    
25. Microsoft JhengHei Light (bold/italic)
    
26. Malgun Gothic Semilight (bold/italic)
    
27. Leelawadee UI Semilight (bold/italic)
    
28. Gloucester MT Extra Condensed (bold/italic)
    
29. Gill Sans Ultra Bold Condensed (bold/italic)
    
30. Gill Sans Ultra Bold (bold/italic)
    
31. Gill Sans MT Ext Condensed Bold (bold/italic)
    
32. Gill Sans MT Condensed (bold/italic)
    
33. Franklin Gothic Medium Cond (bold/italic)
    
34. Franklin Gothic Heavy (bold/italic)
    
35. Franklin Gothic Medium (bold/italic)
    
36. Franklin Gothic Demi (bold/italic)
    
37. Franklin Gothic Demi Cond (bold/italic)
    
38. Franklin Gothic Book (bold/italic)
    
39. Dubai Medium (bold/italic)
    
40. Dubai Light (bold/italic)
    
41. Corbel Light (bold/italic)
    
42. Copperplate Gothic Light (bold/italic)
    
43. Copperplate Gothic Bold (bold/italic)
    
44. Cooper Black (bold/italic)
    
45. Cascadia Mono SemiLight (bold/italic)
    
46. Cascadia Mono SemiBold (italic)
    
47. Cascadia Mono Light (bold/italic)
    
48. Cascadia Code SemiLight (bold/italic)
    
49. Cascadia Mono ExtraLight (bold/italic)
    
50. Cascadia Code SemiBold (italic)
    
51. Cascadia Code Light (bold/italic)
    
52. Cascadia Code ExtraLight (bold/italic)
    
53. Candara Light (bold/italic)
    
54. Calibri Light (bold/italic)
    
55. Britannic Bold (bold/italic)
    
56. Bodoni MT Condensed (bold/italic)
    
57. Bodoni MT Black (italic)
    
58. Bernard MT Condensed (bold/italic)
    
59. Bahnschrift SemiLight Condensed (bold/italic)
    
60. Bahnschrift SemiLight SemiConde (bold/italic)
    
61. Bahnschrift SemiLight (bold/italic)
    
62. Bahnschrift SemiBold (bold/italic)
    
63. Bahnschrift SemiBold SemiConden (bold/italic)
    
64. Bahnschrift SemiCondensed (bold/italic)
    
65. Bahnschrift SemiBold Condensed (bold/italic)
    
66. Bahnschrift Light Condensed (bold/italic)
    
67. Bahnschrift Light  (bold/italic)
    
68. Bahnschrift Condensed (bold/italic)
    
69. Arial Rounded MT Bold (bold/italic)
    
70. Arial Narrow (bold/italic)
    
71. Arial Black (italic)

Document exported to DOCX with 2025 Q2 cannot be opened by 2025 Q1 or previous versions.

Workaround: Use document processing to fix the document.

var processing_provider = new Telerik.Windows.Documents.Flow.FormatProviders.Docx.DocxFormatProvider();

var document = processing_provider.Import(File.ReadAllBytes("C:\\Users\\test\\Downloads\\word1.docx"),null);
var bytes_ = processing_provider.Export(document, null);

var rtb_provider = new Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.DocxFormatProvider();
var doc = rtb_provider.Import(bytes_);
radRichTextBox.Document = doc;

Can be reproduced in the demo:

• Open the default document in the radrichtextbox demo
• Activate change tracking (button "Track Changes")
• Make a modification to the document (delete a paragraph)
• Hover your mouse over the delete text: the date in the revision info. has always ":00" for the seconds

In our processes, it is very important to know if a change happened before or after another action. If all the revisions have zero seconds, it's impossible to determine the order of the changes when they happened in the same minute.

This is not a new bug introduce in SP2: it was there in SP1.

The operating system region and language needs to be set to Japanese.

RichTextBox seems to have several issues with Japanese, but being locked from visibility or edit is the biggest one.

To recreate, start typing in Japanese, push the "right" arrow after selecting the IME option desired, then hit "spacebar." This seems to duplicate the text, then when trying to continue to type into the text field the text is no longer visible (yet the IME does display).

The native WPF RichTextBox has no issues (and also supports partial selection, etc.).

I've attached a video to show the behavior with 2025 Q1 telerik.

*note:* This is on a machine with NVIDIA graphics. I mention this as there may be some issues with RichTextBox and the hardware... When I'm in a virtual machine with a completely Japanese environment and virtual graphics driver, there is NO text visible at all when trying to type into the RichTextBox. Works fine with other controls and wpf RichTextBox.

This causes some problems if the hyperlink is edited and the document is exported and imported again.
Workaround:
private void RadRichTextBox1_DocumentChanged(object sender, EventArgs e)
{
    var document = radRichTextEditor1.Document;

    var fields = document.EnumerateChildrenOfType<FieldRangeStart>();

    foreach (FieldRangeStart item in fields)
    {
        radRichTextEditor1.DeleteAnnotationRange(item);
    }
}

When a new line is placed right after a nbsp, the new line is ignored in the layout. This can be reproduced by importing the following HTML:

First&nbsp;<br/>Second