是否使用c#中的ITextSharp将文本添加到现有的pdf文件？

using iTextSharp.text;
using iTextSharp.text.pdf;

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

Document document = new Document();
PdfCopy copy = new PdfCopy(document, new FileStream("new.pdf", FileMode.Create));
document.Open();

for (int i = 1; i <= reader.NumberOfPages; i++)
{
    PdfImportedPage page = copy.GetImportedPage(reader, i);
    copy.AddPage(page);
}

PdfContentByte cb = copy.DirectContent;
BaseFont font = BaseFont.CreateFont(BaseFont.HELVETICA, BaseFont.CP1252, BaseFont.NOT_EMBEDDED);
cb.BeginText();
cb.SetFontAndSize(font, 12);
cb.ShowTextAligned(Element.ALIGN_LEFT, "Hello, World!", 100, 100, 0);
cb.EndText();

document.Close();