我想在创建新文档时添加现有的 PDF 文件。 答案在这里：

Step1: 
string pdfFilePath = "your path of pdf file";
byte[] bytes = System.IO.File.ReadAllBytes(pdfFilePath);

Step2:
internal static void addPDF(PdfWriter writer,Document document,Byte[] pdf)
    {
        PdfReader reader = new PdfReader(pdf);
        PdfContentByte pcb = writer.DirectContentUnder;
        Int32 pages = reader.NumberOfPages;
        for (Int32 pagenum = 1; pagenum <= pages; pagenum++)
        {
            document.NewPage();
            PdfImportedPage page = writer.GetImportedPage(reader,pagenum);
            // Render their page in our document.
            pcb.AddTemplate(page,0);
        }
    }