private  void MergeDifferentDocumentsPagesWithFixed()
        {
            string[] documentsToMerge =
            {
                "14301-STOCK_Proforma.pdf",
                "14302-STOCK_Proforma.pdf",
                "14303-STOCK_Proforma.pdf",
                "14304-STOCK_Proforma.pdf",
                "14305-STOCK_Proforma.pdf",
                "14330-STOCK_Proforma.pdf"
            };
            RadFixedDocument doc = new RadFixedDocument();
            PdfFormatProvider provider = new PdfFormatProvider();
            foreach (string current_item in documentsToMerge)
            {
                string path = @"..\..\Samples\" + current_item;
                RadFixedDocument docToMerge;
                if (!File.Exists(path))
                {
                    continue;
                }
                using (Stream stream = File.OpenRead(path))
                {
                    docToMerge = provider.Import(stream);
                }
                doc.Merge(docToMerge);
            }


            string outputFilePath = @"..\..\mergedFixed.pdf";
            File.Delete(outputFilePath);

            using (Stream output = File.OpenWrite(outputFilePath))
            {
                provider.Export(doc, output);
            }
            Process.Start(new ProcessStartInfo() { FileName = outputFilePath, UseShellExecute = true });


        }

   private void MergeDifferentDocumentsPages(string resultFileName)
   {
       string[] documentsToMerge =
       {
           "14301-STOCK_Proforma.pdf",
           "14302-STOCK_Proforma.pdf",
           "14303-STOCK_Proforma.pdf",
           "14304-STOCK_Proforma.pdf",
           "14305-STOCK_Proforma.pdf",
           "14330-STOCK_Proforma.pdf"
       };

       File.Delete(resultFileName);
       using (PdfStreamWriter fileWriter = new PdfStreamWriter(File.OpenWrite(resultFileName)))
       {
           foreach (string documentName in documentsToMerge)
           {
               string path = @"..\..\Samples\" + documentName;
               using (PdfFileSource fileToMerge = new PdfFileSource(File.OpenRead(path)))
               {
                   foreach (PdfPageSource pageToMerge in fileToMerge.Pages)
                   {
                       fileWriter.WritePage(pageToMerge);
                   }
               }
           }
       }


      
       Process.Start(new ProcessStartInfo() { FileName = resultFileName, UseShellExecute = true });
   }

PdfFormatProvider provider = new PdfFormatProvider();
provider.ImportSettings.DocumentUnhandledException += this.ImportSettings_DocumentUnhandledException;

private void ImportSettings_DocumentUnhandledException(object sender, DocumentUnhandledExceptionEventArgs e)
{
    if (e.Exception is InvalidGraphicOperandsCountException)
    {
        e.Handled = true;
    }
}

W                          % clipping path operator
0 0 m                     % start of the path construction
596 0 l
596 842 l
0 842 l
h                            % end of the path construction
n                            % path-painting operator

0 0 m
596 0 l
596 842 l
0 842 l
h
W
n

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)