public static void ExportToPdf(DxfModel model) {
            string filename = Path.GetFileName(model.Filename);
            string dir = Path.GetDirectoryName(model.Filename);
            string filenameNoExt = Path.GetFileNameWithoutExtension(filename);
            // as PDF
            using (FileStream stream = File.Create(Path.Combine(dir, filenameNoExt + ".pdf"))) {
                PdfExporter pdfExporter = new PdfExporter(stream);
                pdfExporter.EmbedFonts = true;

                GraphicsConfig config = (GraphicsConfig)GraphicsConfig.AcadLikeWithWhiteBackground.Clone();
                config.DisplayLineTypeElementShapes = true;
                config.TryDrawingTextAsText = true;

                foreach (DxfLayout layout in model.OrderedLayouts) {                    
                    foreach (var item in layout.Viewports)
                    {
                        item.RenderMode = RenderMode.GouraudShaded; // this is the code I added
                    }
                    AddLayoutToPdfExporter(pdfExporter, config, model, null, layout);
                }
                pdfExporter.EndDocument();
            }
        }