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 });
        }

   at Telerik.Windows.Documents.Core.Fonts.Type1.Type1Format.Type1FontSource.GetFontFamily()

This exception was originally thrown at this call stack:
    Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Model.Elements.DocumentStructure.DocumentCatalog.CopyEmbeddedFilesTo(Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Import.Parser.PostScriptReader, Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Import.IRadFixedDocumentImportContext) in DocumentCatalog.cs

if (document.AcroForm.ViewersShouldRecalculateWidgetAppearances)
{
	document.AcroForm.ViewersShouldRecalculateWidgetAppearances = false;
}