RtfFormatProvider provider = new RtfFormatProvider();
var rtf = File.ReadAllText(ofd.FileName);
rtf = this.ReplaceSelfBasedOnStyle(rtf);
this.flowDocument = provider.Import(rtf);

...

private string ReplaceSelfBasedOnStyle(string rtf)
{
    string pattern = @"{[\n]*\\s[0-9]+[^;]* \\\w* (Normal);}";

    var matches = Regex.Matches(rtf, pattern);
    foreach (Match match in matches)
    {
        string oldValue = match.Value;
        string newValue = oldValue.Replace(@"\sbasedon0 ", string.Empty);

        rtf = rtf.Replace(oldValue, newValue);
    }

    return rtf;
}

var rtf = File.ReadAllText(fileName);
rtf = this.RenameStyleDifferentInOnlySpaces(rtf);
var document = provider.Import(rtf);

...

private string RenameStyleDifferentInOnlySpaces(string rtf)
{
    HashSet<string> styles = new HashSet<string>();

    string pattern = @"{\\(?:\*\\c)?s([0-9]+)[^}]*\n?[^}]*\\[^' ]* ?'?([^;]*)";

    var matches = Regex.Matches(rtf, pattern);
    foreach (Match match in matches)
    {
        string styleName = match.Groups[2].Value.Replace(" ", string.Empty);
        if (styles.Contains(styleName))
        {
            styleName = this.ReplaceOldStyleName(ref rtf, styles, match).Replace(" ", string.Empty);
        }

        styles.Add(styleName);
    }

    return rtf;
}

private string ReplaceOldStyleName(ref string rtf, HashSet<string> styles, Match match)
{
    string oldStyleName = match.Groups[2].Value;
    StringBuilder styleNameBuilder = new StringBuilder(oldStyleName + "0");
    while (styles.Contains(styleNameBuilder.ToString().Replace(" ", string.Empty)))
    {
        styleNameBuilder.Append("0");
    }

    string oldMatch = match.Groups[0].Value;
    string newMatch = oldMatch.Replace(oldStyleName, styleNameBuilder.ToString());
    rtf = rtf.Replace(oldMatch, newMatch);
    return styleNameBuilder.ToString();
}

            Telerik.Windows.Documents.Flow.Model.RadFlowDocument document;

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

            using (Stream input = File.OpenRead("PT1987 VU22888 Moodle Specification List [2024032716].docx"))
            {
                document = docXprovider.Import(input);
            }

            string normalStyleId = BuiltInStyleNames.NormalStyleId;
            Style normalStyle = document.StyleRepository.AddBuiltInStyle(normalStyleId);
            normalStyle.ParagraphProperties.SpacingAfter.LocalValue = 0;
            normalStyle.ParagraphProperties.LineSpacing.LocalValue = 1;

        static void Main(string[] args)
        {
                      Telerik.Windows.Documents.Flow.Model.RadFlowDocument templateDocument = GetDocument("Template.rtf");
            Telerik.Windows.Documents.Flow.Model.RadFlowDocument contentDocument = GetDocument("Content.rtf");
            InsertDocumentOptions options = new InsertDocumentOptions();
            options.ConflictingStylesResolutionMode = ConflictingStylesResolutionMode.RenameSourceStyle;
            options.InsertLastParagraphMarker = true;

            RadFlowDocumentEditor editor = new RadFlowDocumentEditor(templateDocument);
            editor.InsertDocument(contentDocument, options);

            string mergedDocumentFilePath ="MergeDocumentsWithWordsProcessing.rtf";
            File.Delete(mergedDocumentFilePath);
            WriteDocToFile(templateDocument, mergedDocumentFilePath);

        }
        private static Telerik.Windows.Documents.Flow.Model.RadFlowDocument GetDocument(string rtfFilePath)
        {
            Telerik.Windows.Documents.Flow.Model.RadFlowDocument document = null;
            var rtfImporter = new Telerik.Windows.Documents.Flow.FormatProviders.Rtf.RtfFormatProvider();
            using (Stream stream = File.OpenRead(rtfFilePath))
            {
                document = rtfImporter.Import(stream);
            }
            return document;
        }


        private static void WriteDocToFile(Telerik.Windows.Documents.Flow.Model.RadFlowDocument doc, string filename)
        {
            var rtfExporter = new Telerik.Windows.Documents.Flow.FormatProviders.Rtf.RtfFormatProvider();
            string rtfText = rtfExporter.Export(doc);
            File.WriteAllText(filename, rtfText);

            Process.Start(filename);
        }