The exception is reproduced only with specific custom fonts and is thrown by the ComputeSubset() method of the GlyphTypeface class with the following message: "file does not conform to the expected file format specification". However, we can handle the exception and export the font data using the base class.

Available in LIB Version 2018.2.820.

When the rtf document contains a table as the one below and the empty declaration is used in the following structure, the document cannot be imported:

{\fonttbl
{\f0 Verdana;}
{\f1 Times New Roman;}
{\f2 ;}
{\f3 Segoe UI;}}

When the first table cell for a table row is vertically merged, the height of the row is exported improperly - instead the height of the row containing the beginning of the merged cell is exported.

The spacing between the letters is too big.

After performance investigation related to feedback item http://feedback.telerik.com/Project/143/Feedback/Details/171108 , one bottleneck was identified as redundant calls to internal methods for getting parent of a document element.

Available in LIB Version 2018.2.820.

In Material theme, RadRichTextBox should be enclosed by border.
In Fluent theme, RadRichTextBox should have shadow.

The problems are more visible when RadRichTextBox.LayoutMode is Flow, but the border/shadow should also be present in Paged layout mode.

Fix available in R3 2018 SP1 release.

When spell checking dialog is used for spell checking, and the user proceeds to the next word (either by changing with a suggestion, ignoring, etc.), the view port is not scrolled to include the next currently spell checked word.

Fix available in R3 2018 Official Release.

Open a docx file in richtextbox, if numpages in headers do any subtraction, it can not display correctly. see the attached pictures for how to display in richtextbox and microsoft word 2016.

When RTF documents are imported simultaneously in different threads, sometimes ArgumentException with message "An item with the same key has already been added" is thrown with the following call stack:

at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
   at System.Collections.Generic.Dictionary`2.Add(TKey key, TValue value)
   at Telerik.Windows.Documents.FormatProviders.ReplaceStyleNameHandler.Add(String displayName, String name)
   at Telerik.Windows.Documents.FormatProviders.ReplaceStyleNameHandler.InitializeStyleDisplayNameToStyleNameDictionary()
   at Telerik.Windows.Documents.FormatProviders.ReplaceStyleNameHandler.get_StyleDisplayNameToStyleName()
   at Telerik.Windows.Documents.FormatProviders.ReplaceStyleNameHandler.TryGetStyleNamesByStyleName(String displayName, RadDocumentDefaultStyleInfo& defaultStyleInfo)
   at Telerik.Windows.Documents.FormatProviders.ReplaceStyleNameHandler.Replace(ReplaceStyleNameContext context)
   at Telerik.Windows.Documents.FormatProviders.Rtf.Import.RtfStyleImporter.DoVisitText(RtfText text)
   at Telerik.Windows.Documents.FormatProviders.Rtf.Import.RtfElementIteratorBase.VisitElement(RtfElement element, Boolean recursive)
   at Telerik.Windows.Documents.FormatProviders.Rtf.Import.RtfElementIteratorBase.VisitGroupChildren(RtfGroup group, Boolean recursive)
   at Telerik.Windows.Documents.FormatProviders.Rtf.Import.RtfStyleImporter.ImportStyleGroup(RtfGroup group)
   at Telerik.Windows.Documents.FormatProviders.Rtf.Import.RtfStylesTable.FillStyle(RtfGroup group, StyleType type, Boolean isDefault)
   at Telerik.Windows.Documents.FormatProviders.Rtf.Import.RtfStylesTable.DoVisitGroup(RtfGroup group)
   at Telerik.Windows.Documents.FormatProviders.Rtf.Import.RtfElementIteratorBase.VisitElement(RtfElement element, Boolean recursive)
   at Telerik.Windows.Documents.FormatProviders.Rtf.Import.RtfElementIteratorBase.VisitGroupChildren(RtfGroup group, Boolean recursive)
   at Telerik.Windows.Documents.FormatProviders.Rtf.Import.RtfStylesTable.ReadTable(RtfGroup group, RtfImportContext context)
   at Telerik.Windows.Documents.FormatProviders.Rtf.Import.RtfHandlers.RtfGroupHandlers.StylesTableHandler(RtfGroup group, RtfImportContext context)
   at Telerik.Windows.Documents.FormatProviders.Rtf.Import.RtfDocumentImporter.DoVisitGroup(RtfGroup group)
   at Telerik.Windows.Documents.FormatProviders.Rtf.Import.RtfElementIteratorBase.VisitElement(RtfElement element, Boolean recursive)
   at Telerik.Windows.Documents.FormatProviders.Rtf.Import.RtfElementIteratorBase.VisitGroupChildren(RtfGroup group, Boolean recursive)
   at Telerik.Windows.Documents.FormatProviders.Rtf.Import.RtfDocumentImporter.ImportRoot(RtfGroup rtfGroup)
   at Telerik.Windows.Documents.FormatProviders.Rtf.Import.RtfDocumentImporter.Import(Stream input, RtfImportSettings settings)
   at Telerik.Windows.Documents.FormatProviders.Rtf.RtfFormatProvider.Import(Stream input)
   at Telerik.Windows.Documents.FormatProviders.Rtf.RtfFormatProvider.Import(String input)

Fix available in LIB Version 2018.3.924.

When a data provider is used, the designer does not show the RadRichTextBox content, that is shown with no problem at run time.

Workaround: The issue is with finding the format provider used by the data provider. This issue appears at design time only. The workaround is to manually set the format provider of the data provider:
        <telerik:HtmlDataProvider
            x:Name="htmlDataProvider"
            RichTextBox="{Binding ElementName=radRichTextBox, Mode=OneWay}"
            Html="&lt;h1&gt;Header3&lt;/h1&gt;">
            <telerik:HtmlDataProvider.FormatProvider>
                <telerik:HtmlFormatProvider />
            </telerik:HtmlDataProvider.FormatProvider>
        </telerik:HtmlDataProvider>

Whenever the focus is on RadRichTextBox, the keyboard is displayed with Uppercase by Default. After inputting 2-3 characters in uppercase, it changes to lowercase and then alternates between uppercase and lower per character.

Import description lists: <dl>, <dt>, <dd> tags.

Possible way for implementation is with paragraphs with left margin.

When a separate file for setting the xaml styles is used and then this file is merged in App.xaml the style cannot be found. 
As a workaround merge the resources directly in the App.xaml.

Fix available in LIB Version 2018.3.1029.

When a new document is created and GetStatisticsInfo is called on it, a NullReferenceException is thrown.

The issue is a regression introduced with another fix, released in R2 2018.

Some of the formatting symbols (e.g. space, tab) don't affect the height of the paragraph. With the current implementation, changing the font size of a formatting symbol, which is first in the paragraph, changes its height as well.

When the document is protected, RadRichTextBox.CommandExecuting is not raised when the user double clicks the header or footer area.

When the MoveToDocumentElementStart is called with parameter that is PermissionRangeStart and then is called a second time, the position moves to the previous position instead.

If you insert a picture in a RichTextBox in web layout mode and stretch it to exceed the boundaries of the control (set a smaller size to make it more convenient), a horizontal scrollbar will appear. If you do the same, but with the picture in a table, the scrollbar will not appear.

When you first insert a table, and try to grab the right border and resize it, the result is that the inner cells resize, while the total width of the table remains the same. If you move the inner borders a bit and then try again to resize the whole table, it will work.