到2011年8月iTextSharp最新版本下載地址：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using iTextSharp;
using iTextSharp.text;
using iTextSharp.text.pdf;
using System.IO;
using System.Diagnostics;

//加密程式碼

 protected void Button1_Click(object sender, EventArgs e)
        {
            string PdfPath = AppDomain.CurrentDomain.SetupInformation.ApplicationBase;
            PdfPath = PdfPath + "PDFFiles\\";
            string PdfFIle = PdfPath + "008.PDF";

            string sname = PdfFIle;//要加密的檔案
            string sname1 = PdfPath + "test.PDF";//加密後生成的檔案

            PdfReader reader = new PdfReader(sname);
            int n = reader.NumberOfPages;

            Document document = new Document(reader.GetPageSizeWithRotation(1));
            PdfWriter writer = PdfWriter.GetInstance(document, new FileStream(sname1, FileMode.Create));
            writer.SetEncryption(PdfWriter.STRENGTH128BITS, "123456", null, PdfWriter.AllowPrinting);

            document.Open();
            PdfContentByte cb = writer.DirectContent;
            PdfImportedPage page;
            int rotation;
            int i = 0;
            // step 4: we add content
            while (i < n)
            {
                i++;
                document.SetPageSize(reader.GetPageSizeWithRotation(i));
                document.NewPage();
                page = writer.GetImportedPage(reader, i);
                rotation = reader.GetPageRotation(i);
                if (rotation == 90 || rotation == 270)
                {
                    cb.AddTemplate(page, 0, -1f, 1f, 0, 0, reader.GetPageSizeWithRotation(i).Height);
                }
                else
                {
                    cb.AddTemplate(page, 1f, 0, 0, 1f, 0, 0);
                }

            }
            //
            // step 5: we close the document

            document.Close();
            writer.Close();
        }

說明：加密後的檔案沒有訪問，但PDF閱讀器左側的總頁數似乎有點問題，不影響使用。

有人解決的可以跟上。