Workaround: use a custom RichTextEditorRibbonBar

Public Class CustomRichTextEditorRibbonBar
Inherits RichTextEditorRibbonBar

    Dim headerText As String
    Protected Overrides Function GetInsertTableItems() As RadItemCollection
        Dim collection As RadItemCollection = MyBase.GetInsertTableItems()
        Dim headerItem As RadMenuInsertTableItem = TryCast(collection(0), RadMenuInsertTableItem)
        headerText = "My Header Text"
        Dim fi As FieldInfo = GetType(RadMenuInsertTableItem).GetField("header", BindingFlags.NonPublic Or BindingFlags.Instance)
        Dim header As LightVisualElement = fi.GetValue(headerItem)
        header.Text = headerText
        AddHandler header.TextChanged, AddressOf header_TextChanged
        Return collection
    End Function

    Private Sub header_TextChanged(sender As Object, e As EventArgs)
        Dim lve As LightVisualElement = TryCast(sender, LightVisualElement)
        If lve.Text = "Insert Table" Then
            lve.Text = headerText
        End If
    End Sub
End Class

Workaround:

private void CopyButton_Click(object sender, EventArgs e)
{
    this.radRichTextEditor1.Document.EnsureDocumentMeasuredAndArranged();
    if (this.radRichTextEditor1.Document.Selection.IsEmpty)
    {
        return;
    }

    string selectedText = this.radRichTextEditor1.Document.Selection.GetSelectedText();
    DocumentFragment fragmentToCopy = this.radRichTextEditor1.Document.Selection.CopySelectedDocumentElements(true);
    DataObject dataObject = new DataObject();
    if (selectedText != "")
    {
        ClipboardEx.SetText(null, selectedText, dataObject);
    }

    ClipboardEx.SetDocument(fragmentToCopy, dataObject);
    ClipboardEx.SetDataObject(dataObject);
}

To reproduce: please refer to the attached sample project.

Workaround: export RadRichTextEditor's content to a .doc file. Then, use the RadWordsProcessing library to import the .doc file and export it as a pdf:

Telerik.WinForms.Documents.FormatProviders.OpenXml.Docx.DocxFormatProvider provider = 
    new Telerik.WinForms.Documents.FormatProviders.OpenXml.Docx.DocxFormatProvider();
string fileName = @"..\..\exported.doc";
string pdfFileName = @"..\..\exported.pdf";
Stream s = new FileStream(fileName,FileMode.Create, FileAccess.Write);
provider.Export(document, s);
s.Close();
s.Dispose();

Telerik.Windows.Documents.Flow.FormatProviders.Docx.DocxFormatProvider provider2 = 
    new Telerik.Windows.Documents.Flow.FormatProviders.Docx.DocxFormatProvider();
using (Stream input = File.OpenRead(fileName))
{
    Telerik.Windows.Documents.Flow.Model.RadFlowDocument document2 = provider2.Import(input);
    Telerik.Windows.Documents.Flow.FormatProviders.Pdf.PdfFormatProvider provider3 = 
        new Telerik.Windows.Documents.Flow.FormatProviders.Pdf.PdfFormatProvider();
    using (Stream output = File.OpenWrite(pdfFileName))
    { 
        provider3.Export(document2, output);
    }
}

System.Diagnostics.Process.Start(pdfFileName);       

To reproduce:
Seth the default font like this:

radRichTextEditor1.Document.StyleRepository[RadDocumentDefaultStyles.NormalStyleName].SpanProperties.FontFamily = new Telerik.WinControls.RichTextEditor.UI.FontFamily("Segoe Script");
          
radRichTextEditor1.RichTextBoxElement.ChangeFontFamily(new Telerik.WinControls.RichTextEditor.UI.FontFamily("Segoe Script"));
radRichTextEditor1.RichTextBoxElement.ChangeFontSize(Unit.PointToDip(12));
radRichTextEditor1.RichTextBoxElement.ChangeFontStyle(Telerik.WinControls.RichTextEditor.UI.FontStyles.Italic);
radRichTextEditor1.RichTextBoxElement.ChangeFontWeight(Telerik.WinControls.RichTextEditor.UI.FontWeights.Bold);

Start the application and insert header footer and endnotes.

To reproduce:

private void radButton1_Click(object sender, EventArgs e)
{
    var document = new RadDocument(); 
    var section = new Section();
    document.Sections.Add(section);
    var paragraph = new Paragraph();
    section.Blocks.Add(paragraph);
    var span = new Span("BOLD");
    span.FontWeight = FontWeights.Bold;
    paragraph.Inlines.Add(span);

    span = new Span(" REGULAR");
    paragraph.Inlines.Add(span);

    editor.Document = document;

    var pdfExporter = new PdfFormatProvider();
    var path = Path.GetTempFileName() + ".pdf";
    using (var file = File.Create(path))
    {
        pdfExporter.Export(document, file);
        file.Flush();
        file.Close();
        
    }
    Process.Start(path);
}

Workaround: export .docx file and use the RadWordsProcessing library to import the .doc file and export it to pdf:

private void radButton1_Click(object sender, EventArgs e)
{
    var document = new RadDocument();
    var section = new Section();
    document.Sections.Add(section);
    var paragraph = new Paragraph();
    section.Blocks.Add(paragraph);
    var span = new Span("BOLD");
    span.FontWeight = FontWeights.Bold;
    paragraph.Inlines.Add(span);

    span = new Span(" REGULAR");
    paragraph.Inlines.Add(span);

    editor.Document = document;

    var wordExporter = new Telerik.WinForms.Documents.FormatProviders.OpenXml.Docx.DocxFormatProvider();
    var wordPath = Path.GetTempFileName() + ".doc";
    using (var file = File.Create(wordPath))
    {
        wordExporter.Export(document, file);
        file.Flush();
        file.Close();
    }

    Telerik.Windows.Documents.Flow.FormatProviders.Docx.DocxFormatProvider provider =
        new Telerik.Windows.Documents.Flow.FormatProviders.Docx.DocxFormatProvider();
    using (Stream input = File.OpenRead(wordPath))
    {
        Telerik.Windows.Documents.Flow.Model.RadFlowDocument doc = provider.Import(input);

        Telerik.Windows.Documents.Flow.FormatProviders.Pdf.PdfFormatProvider pdfProvider =
            new Telerik.Windows.Documents.Flow.FormatProviders.Pdf.PdfFormatProvider();
        string filePath = @"..\..\" + DateTime.Now.ToLongTimeString().Replace(":", "-");
        using (var output = File.Create(filePath))
        {
            pdfProvider.Export(doc, output);
        }
        Process.Start(filePath);
    }
}

When the specified length of the stream is larger than the actual one, Adobe throws errors and removes the image.

Workaround:

Set the format provider settings:

PdfFormatProvider provider = new PdfFormatProvider();
PdfExportSettings exportSettings = new PdfExportSettings();
exportSettings.ContentsDeflaterCompressionLevel = 9;
exportSettings.ImagesDeflaterCompressionLevel = 9;
provider.ExportSettings = exportSettings;

To reproduce:
1. Insert a table
2. Select all cells in it
3. Press the Backspace key => the table is removed but the table adorner is still visible

The adorner disappears if clicked

To reproduce:
- Add some misspelled words.
- Enable the spell check - the layout is updated and the words are underlined.
- Disable the spell check - the lines are not removed until one clicks in the RichTextEditor.

Workaround:
 radRichTextEditor1.IsSpellCheckingEnabled = !radRichTextEditor1.IsSpellCheckingEnabled;
 radRichTextEditor1.Document.LayoutMode = DocumentLayoutMode.Paged;
 radRichTextEditor1.Document.LayoutMode = DocumentLayoutMode.Flow;

 html tables cannot be properly pasted in word when imported table size is defined in "pt".

consider adding the a table like this as well:
<html>
<body>
<table border="1">
	<tbody>
		<tr>
			<td>A</td>
			<td>B</td>
			<td>C</td>					
		</tr>	
		<tr>
			<td>A1</td>
			<td>B1</td>
			<td>C1</td>			
		</tr>	
	</tbody>
</table>
</body>
</html>

To reproduce:
- Paste some text with a barcode font in a RadRichTextBox.
- Export to a pdf document.
- In the pdf document the text is not displayed as barcode.

Workaround:
string styleSuffix = "_1";
foreach (var importedStyle in rtfDoc.StyleRepository)
{
        importedStyle.Name = string.Concat(importedStyle.Name, styleSuffix);
}

Invisible borders in html format are imported as black borders in RadRichTextBox

Resolution: 
This issue is addressed in the new version of the control - RadRichTextEditor. Please use the new control instead the RadRichTextBox. 

To reproduce:
- insert a fragmet from another RichtextEditor like this:
private void Button_Click(object sender, RoutedEventArgs e)
{
radRichTextBox.Document.Sections.Clear();
radRichTextBox.Document.Sections.Add(new Section());

radRichTextBox.Document.CaretPosition.MoveToLastPositionInDocument(); 
radRichTextBox.DocumentEditor.InsertParagraph();
radRichTextBox.DocumentEditor.InsertFragment(new DocumentFragment(radRichTextEditor1.Document));
radRichTextBox.DocumentEditor.InsertParagraph();
}

Workaround:
There is no point of resetting the section in such way. Nevertheless, you can avoid the exception by using one of the following methods:

radRichTextBox.Document.MeasureAndArrangeInDefaultSize();
radRichTextBox.UpdateEditorLayout();

To reproduce:
- Copy the content from the attached document - do not copy the last row.
- Remove the bookmarks like this:

bookmarks = radRichTextEditor1.Document.GetAllBookmarks().ToArray();
foreach (BookmarkRangeStart item in bookmarks)
{
    radRichTextEditor1.Document.GoToBookmark(item);
    radRichTextEditor1.DocumentEditor.DeleteBookmark(item);
   
}

Workaround:
                    var invalidStarts = this.radRichTextBox.Document
                    .EnumerateChildrenOfType<BookmarkRangeStart>()
                    .Where(start => start.End == null)
                    .ToList();

                    foreach (var invalidStart in invalidStarts)
                    {
                        invalidStart.Parent.Children.Remove(invalidStart);
                    } 
Note that when this workaround is used the document history will be deleted as well, which means that the undo operation will no longer hold information for previous operations

Workaround:
public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();

        this.radRichTextEditor1.IsSpellCheckingEnabled = true;

        this.radRichTextEditor1.Insert("SOooome wrrrrong wooooooooords.");

        this.radRichTextEditor1.CommandExecuted += radRichTextEditor1_CommandExecuted;
    }

    bool shouldProcess = true;
    private void radRichTextEditor1_CommandExecuted(object sender, Telerik.WinForms.Documents.RichTextBoxCommands.CommandExecutedEventArgs e)
    {
        if (e.Command is DeleteCommand)
        {
            if (shouldProcess)
            {
                shouldProcess = false;
                RichTextEditorInputBehavior behavior = this.radRichTextEditor1.InputHandler;
                behavior.ProcessKeyDown(new KeyEventArgs(Keys.Back));    
            }

            shouldProcess = true;
        }
    }
}