如何使用C#中的IText7在现有pdf的顶部添加条形码？

using iText.Kernel.Pdf;
using iText.Layout;
using iText.Layout.Element;
using iText.Layout.Properties;
using iText.Barcodes;

PdfDocument pdfDoc = new PdfDocument(new PdfReader("existing.pdf"));
PdfDocument finalPdfDoc = new PdfDocument(new PdfWriter("final.pdf"));

PdfPage page = pdfDoc.GetPage(1);
PdfPage finalPage = finalPdfDoc.AddNewPage();

Document doc = new Document(finalPage, page.GetPageSize());
PdfCanvas canvas = new PdfCanvas(finalPage);
PdfFormXObject pageCopy = page.CopyAsFormXObject(finalPdfDoc);
canvas.AddXObject(pageCopy, 0, 0);

BarcodeEAN barcode = new BarcodeEAN(finalPdfDoc);
barcode.SetCodeType(BarcodeEAN.EAN13);
barcode.SetCode("1234567890123");

Canvas canvas = new Canvas(finalPage, finalPdfDoc.GetDefaultPageSize())
    .SetFontSize(12)
    .SetWidth(100)
    .SetTextAlignment(TextAlignment.CENTER)
    .SetHorizontalAlignment(HorizontalAlignment.CENTER);

Image barcodeImage = new Image(barcode.CreateFormXObject(finalPdfDoc))
    .SetAutoScale(true);

canvas.Add(barcodeImage);
canvas.Close();

doc.Close();
finalPdfDoc.Close();
pdfDoc.Close();