如何编辑现有的pdf文件itext7，C#？

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

PdfReader reader = new PdfReader("existing.pdf");
PdfDocument document = new PdfDocument(reader);

Document doc = new Document(document);
doc.Add(new Paragraph("Hello, World!"));

Image image = new Image(ImageDataFactory.Create("image.jpg"));
doc.Add(image);

Table table = new Table(3);
table.AddCell("Cell 1");
table.AddCell("Cell 2");
table.AddCell("Cell 3");
doc.Add(table);

doc.Close();