System.OutOfMemoryException: Insufficient memory to continue the execution of the program.
   at System.Text.StringBuilder.ExpandByABlock(Int32 minBlockCharCount)
   at System.Text.StringBuilder.AppendWithExpansion(Char value)
   at System.Text.StringBuilder.Append(Char value)
   at Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Utilities.CrossReferenceCollectionReader.GetAllText(Reader reader, Int64 minOffset, Int64 maxOffset)
   at Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Utilities.CrossReferenceCollectionReader.FindAllObjectOffsets(Reader reader, Dictionary`2 tokenToOffsets, Int64 minOffset, Int64 maxOffset)

            RadFlowDocument flowDocument = new RadFlowDocument();
            RadFlowDocumentEditor editor = new RadFlowDocumentEditor(flowDocument);
            Bookmark bookmark = new Bookmark(flowDocument, "Name");
            editor.InsertInline(bookmark.BookmarkRangeStart);

            Table table = editor.InsertTable(1, 2);
            TableCell cell1 = table.Rows[0].Cells[0];
            Paragraph cell_paragraph1 = cell1.Blocks.AddParagraph();
            editor.MoveToParagraphStart(cell_paragraph1);
            editor.InsertText("cell content 01");

            TableCell cell2 = table.Rows[0].Cells[1];
            Paragraph cell_paragraph2 = cell2.Blocks.AddParagraph();
            editor.MoveToParagraphStart(cell_paragraph2);
            editor.InsertText("cell content 02");

            editor.MoveToTableEnd(table);
            editor.InsertInline(bookmark.BookmarkRangeEnd); 

            RadFlowDocument newFlowDocument = new RadFlowDocument();
            RadFlowDocumentEditor editor02 = new RadFlowDocumentEditor(newFlowDocument);
            editor.InsertDocument(flowDocument);

            RadFlowDocument flowDocument = new RadFlowDocument();
            RadFlowDocumentEditor editor = new RadFlowDocumentEditor(flowDocument);
            //Bookmark bookmark = new Bookmark(flowDocument, "Name");
            //editor.InsertInline(bookmark.BookmarkRangeStart);

            editor.InsertBookmark("before");

            Table table = editor.InsertTable(1, 2);
            TableCell cell1 = table.Rows[0].Cells[0];
            Paragraph cell_paragraph1 = cell1.Blocks.AddParagraph();
            editor.MoveToParagraphStart(cell_paragraph1);
            editor.InsertText("cell content 01");

            TableCell cell2 = table.Rows[0].Cells[1];
            Paragraph cell_paragraph2 = cell2.Blocks.AddParagraph();
            editor.MoveToParagraphStart(cell_paragraph2);
            editor.InsertText("cell content 02");

            editor.MoveToTableEnd(table);

            editor.InsertBookmark("after");

            //editor.InsertInline(bookmark.BookmarkRangeEnd); 

            RadFlowDocument newFlowDocument = new RadFlowDocument();
            RadFlowDocumentEditor editor02 = new RadFlowDocumentEditor(newFlowDocument);
            editor.InsertDocument(flowDocument);

void WriteToPdf(RadFlowDocument document, Stream outputStream) {

   PdfFormatProvider pdfWriter = new() {};

   pdfWriter.Export(document, outputStream);
}

Parallel.ForEachAsync(listOfDocuments, (document, _) => {
   WriteToPdf(document, Stream.Null);
});

System.InvalidOperationException: Operations that change non-concurrent collections must have exclusive access. A concurrent update was performed on this collection and corrupted its state. The collection's state is no longer correct.
   at System.Collections.Generic.Dictionary`2.FindValue(TKey key)
   at Telerik.Windows.Documents.Fixed.Model.Fonts.FontsRepository.TryCreateFont(FontFamily fontFamily, FontStyle fontStyle, FontWeight fontWeight, FontBase& font)
   at Telerik.Windows.Documents.Flow.FormatProviders.Pdf.Utils.Extensions.CopyPropertiesFrom(CharacterProperties fixedProperties, PdfExportContext context, CharacterProperties properties)
   at Telerik.Windows.Documents.Flow.FormatProviders.Pdf.Export.PdfExporter.CreateListLevel(ListLevel flowLevel)
   at Telerik.Windows.Documents.Flow.FormatProviders.Pdf.Export.PdfExporter.CreateList(List flowList)
   at Telerik.Windows.Documents.Flow.FormatProviders.Pdf.Export.PdfExporter.ExportDocument(RadFlowDocument document, RadFixedDocumentEditor editor)
   at Telerik.Windows.Documents.Flow.FormatProviders.Pdf.Export.PdfExporter.ExportInternal()
   at Telerik.Windows.Documents.Common.FormatProviders.FormatProviderBase`1.Export(T document, Stream output)
   at xxxxx.Application.Common.PdfGeneration.PdfWriter.WriteToPdf(RadFlowDocument document, Stream outputStream) 

        static void Main(string[] args)
        {
            string filePath = "Lorem ipsum dolor sit amet.pdf";
            //load a random document
            PdfFormatProvider provider = new PdfFormatProvider();
            RadFixedDocument originalDocument;
            using (Stream stream = File.OpenRead(filePath))
            {
                originalDocument = provider.Import(stream);
            }
            //draw something on the first page
            FixedContentEditor editor = new FixedContentEditor(originalDocument.Pages[0]);
            editor.GraphicProperties.IsFilled = true;
            editor.GraphicProperties.FillColor = RgbColors.Black;
            Telerik.Documents.Primitives.Rect Rect = new Telerik.Documents.Primitives.Rect(10, 10, 200, 100);
            editor.DrawRectangle(Rect);

            //export the pages as images and build a brand new document from the images
            SkiaImageFormatProvider imageProvider = new SkiaImageFormatProvider();
            imageProvider.ExportSettings.ImageFormat = SkiaImageFormat.Jpeg;
            imageProvider.ExportSettings.ScaleFactor = 0.8;
            imageProvider.ExportSettings.Quality = 80;


            RadFixedDocument doc = new RadFixedDocument();
            foreach (RadFixedPage page in originalDocument.Pages)
            {
                byte[] resultImage = imageProvider.Export(page);
                RadFixedPage pdfpage = doc.Pages.AddPage();
                editor = new FixedContentEditor(pdfpage);
                Stream imageStream = new MemoryStream(resultImage);
                editor.DrawImage(imageStream);
            }

            //export the pdf built from the images
            PdfFormatProvider pdfFormatProvider = new PdfFormatProvider();
            string outputPdf = @"output.pdf";
            File.Delete(outputPdf);
            using (Stream output = File.OpenWrite(outputPdf))
            {
                pdfFormatProvider.Export(doc, output);
            }
            Process.Start(new ProcessStartInfo() { FileName = outputPdf, UseShellExecute = true });
        }

        static void Main(string[] args)
        {
            string filePath = "Lorem ipsum dolor sit amet.pdf";
            //load a random document
            PdfFormatProvider provider = new PdfFormatProvider();
            RadFixedDocument originalDocument;
            using (Stream stream = File.OpenRead(filePath))
            {
                originalDocument = provider.Import(stream);
            }
            //draw something on the first page
            FixedContentEditor editor = new FixedContentEditor(originalDocument.Pages[0]);
            editor.GraphicProperties.IsFilled = true;
            editor.GraphicProperties.FillColor = RgbColors.Black;
            Telerik.Documents.Primitives.Rect Rect = new Telerik.Documents.Primitives.Rect(10, 10, 200, 100);
            editor.DrawRectangle(Rect);

            using (Stream output = File.OpenWrite(filePath))
            {
                provider.Export(originalDocument, output);
            }

            using (Stream stream = File.OpenRead(filePath))
            {
                originalDocument = provider.Import(stream);
            }
            //export the pages as images and build a brand new document from the images
            SkiaImageFormatProvider imageProvider = new SkiaImageFormatProvider();
            imageProvider.ExportSettings.ImageFormat = SkiaImageFormat.Jpeg;
            imageProvider.ExportSettings.ScaleFactor = 0.8;
            imageProvider.ExportSettings.Quality = 80;


            RadFixedDocument doc = new RadFixedDocument();
            foreach (RadFixedPage page in originalDocument.Pages)
            {
                byte[] resultImage = imageProvider.Export(page);
                RadFixedPage pdfpage = doc.Pages.AddPage();
                editor = new FixedContentEditor(pdfpage);
                Stream imageStream = new MemoryStream(resultImage);
                editor.DrawImage(imageStream);
            }

            //export the pdf built from the images
            PdfFormatProvider pdfFormatProvider = new PdfFormatProvider();
            string outputPdf = @"output.pdf";
            File.Delete(outputPdf);
            using (Stream output = File.OpenWrite(outputPdf))
            {
                pdfFormatProvider.Export(doc, output);
            }
            Process.Start(new ProcessStartInfo() { FileName = outputPdf, UseShellExecute = true });
        }

        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);
        }

            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;

Stopwatch sw = new Stopwatch();
sw.Start();
IWorkbookFormatProvider formatProvider = new Telerik.Windows.Documents.Spreadsheet.FormatProviders.OpenXml.Xlsx.XlsxFormatProvider();
Workbook workbook = new Workbook();
Worksheet worksheet = workbook.Worksheets.Add();
Worksheet worksheet2 = workbook.Worksheets.Add();
worksheet2.Name ="Days";
List<string> weekdays = new List<string>() { "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"};
for (int i = 0; i < 7; i++)
{
    worksheet2.Cells[0, i].SetValue(weekdays[i]);
}

for (int i = 0; i < 200; i++)
{
    for (int j = 0; j < 10; j++)
    {

        CellIndex cellIndex = new CellIndex(i, j);
        CellSelection selection = worksheet.Cells[cellIndex];
        selection.SetValue("Wednesday");

        var context = new ListDataValidationRuleContext(worksheet, cellIndex)
        {
            InputMessageTitle = "InputMessageTitle",
            InputMessageContent = "InputMessageTitle"
        };

        context.ErrorStyle = ErrorStyle.Stop;
        context.ErrorAlertTitle = "ErrorAlertTitle";
        context.ErrorAlertContent = "ErrorAlertContent";
        context.InCellDropdown = true;
        context.Argument1 = "=Days!A0:A6"; //"Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday";
        ListDataValidationRule rule = new(context);
        worksheet.Cells[cellIndex].SetDataValidationRule(rule);
    }
}

string outputFile = @"..\..\..\output.xlsx";
File.Delete(outputFile);

using (Stream output = new FileStream(outputFile, FileMode.Create))
{

    formatProvider.Export(workbook, output);
}
sw.Stop();
Debug.WriteLine("Export " + sw.ElapsedMilliseconds);

<ol type="a">
<li>
<div><p>WordsProcessing</p>
</div></li>
<li>
<div>
<p>SpreadProcessing</p>
</div>
</li>
<li>
<div>
<p>PdfProcessing</p>
</div>
</li>
</ol>