iTextSharp,a .NET PDF library 。iTextSharp.dll 下載地址 https://sourceforge.net/projects/itextsharp/

iText is a PDF library that allows you to CREATE, ADAPT, INSPECT and MAINTAIN documents in the Portable Document Format (PDF):
- Generate documents and reports based on data from an XML file or a database
- Create maps and books, exploiting numerous interactive features available in PDF
- Add bookmarks, page numbers, watermarks, and other features to existing PDF documents
- Split or concatenate pages from existing PDF files
- Fill out interactive forms
- Serve dynamically generated or manipulated PDF documents to a web browser

iText is used by Java, .NET, Android and GAE developers to enhance their applications with PDF functionality.
iTextSharp is the .NET port.

Several iText engineers are actively supporting the project on on StackOverflow:

http://stackoverflow.com/questions/tagged/itext

http://github.com/itext

以下轉自 http://www.cnblogs.com/LL-723/archive/2013/12/25/3490699.html （先引入itextsharp.dll ， 5.5.9.0版大小是3.86M）

public class PDFSetWaterMark
    {
        /// <summary>
        /// 建立一個顯示指定圖片的pdf
        /// </summary>
        /// <param name="picPdfPath"></param>
        /// <param name="picPath"></param>
        /// <returns></returns>
        public static bool CreatePDFByPic(string picPdfPath,string picPath)
        {
            //新建一個文件
            Document doc = new Document();
            try
            {
                //建立一個書寫器(Writer)與document物件關聯
                PdfWriter.GetInstance(doc, new FileStream(picPdfPath, FileMode.Create, FileAccess.ReadWrite));
                //開啟一個文件
                doc.Open();
                //向文件中新增內容
                Image img = Image.GetInstance(picPath);
                //img.SetAbsolutePosition();
                doc.Add(img);
                return true;
            }
            catch (Exception ex)
            {
                return false;
                throw ex;
            }
            finally
            {
                if (doc != null)
                {
                    doc.Close();
                }
            }

        }

        /// <summary>
        /// 加圖片水印
        /// </summary>
        /// <param name="inputfilepath"></param>
        /// <param name="outputfilepath"></param>
        /// <param name="ModelPicName"></param>
        /// <param name="top"></param>
        /// <param name="left"></param>
        /// <returns></returns>
        public static bool PDFWatermark(string inputfilepath, string outputfilepath, string ModelPicName, float top, float left)
        {
            //throw new NotImplementedException();
            PdfReader pdfReader = null;
            PdfStamper pdfStamper = null;
            try
            {
                pdfReader = new PdfReader(inputfilepath);

                int numberOfPages = pdfReader.NumberOfPages;

                iTextSharp.text.Rectangle psize = pdfReader.GetPageSize(1);

                float width = psize.Width;

                float height = psize.Height;

                pdfStamper = new PdfStamper(pdfReader, new FileStream(outputfilepath, FileMode.Create));

                PdfContentByte waterMarkContent;

                iTextSharp.text.Image image = iTextSharp.text.Image.GetInstance(ModelPicName);

                image.GrayFill = 20;//透明度，灰色填充
                //image.Rotation//旋轉
                //image.RotationDegrees//旋轉角度
                //水印的位置 
                if (left < 0)
                {
                    left = width/2 - image.Width + left;
                }

                //image.SetAbsolutePosition(left, (height - image.Height) - top);
                image.SetAbsolutePosition(left, (height/2 - image.Height) - top);


                //每一頁加水印,也可以設定某一頁加水印 
                for (int i = 1; i <= numberOfPages; i++)
                {
                    //waterMarkContent = pdfStamper.GetUnderContent(i);//內容下層加水印
                    waterMarkContent = pdfStamper.GetOverContent(i);//內容上層加水印

                    waterMarkContent.AddImage(image);
                }
                //strMsg = "success";
                return true;
            }
            catch (Exception ex)
            {
                throw ex;
                
            }
            finally
            {

                if (pdfStamper != null)
                    pdfStamper.Close();

                if (pdfReader != null)
                    pdfReader.Close();
            }
        }
        /// <summary>
        /// 新增普通偏轉角度文字水印
        /// </summary>
        /// <param name="inputfilepath"></param>
        /// <param name="outputfilepath"></param>
        /// <param name="waterMarkName"></param>
        /// <param name="permission"></param>
        public static void setWatermark(string inputfilepath, string outputfilepath,string waterMarkName)
        {
            PdfReader pdfReader = null;
            PdfStamper pdfStamper = null;
            try
            {
                pdfReader = new PdfReader(inputfilepath);
                pdfStamper = new PdfStamper(pdfReader, new FileStream(outputfilepath, FileMode.Create));
                int total = pdfReader.NumberOfPages + 1;
                iTextSharp.text.Rectangle psize = pdfReader.GetPageSize(1);
                float width = psize.Width;
                float height = psize.Height;
                PdfContentByte content;
                BaseFont font = BaseFont.CreateFont(@"C:\WINDOWS\Fonts\SIMFANG.TTF", BaseFont.IDENTITY_H, BaseFont.EMBEDDED);
                PdfGState gs = new PdfGState();
                for (int i = 1; i < total; i++) 
                {
                    content = pdfStamper.GetOverContent(i);//在內容上方加水印
                    //content = pdfStamper.GetUnderContent(i);//在內容下方加水印
                    //透明度
                    gs.FillOpacity = 0.3f;
                    content.SetGState(gs);
                    //content.SetGrayFill(0.3f);
                    //開始寫入文字
                    content.BeginText();
                    content.SetColorFill(BaseColor.LIGHT_GRAY);
                    content.SetFontAndSize(font, 100);
                    content.SetTextMatrix(0, 0);
                    content.ShowTextAligned(Element.ALIGN_CENTER, waterMarkName, width / 2 - 50,height / 2 - 50, 55);
                    //content.SetColorFill(BaseColor.BLACK);
                    //content.SetFontAndSize(font, 8);
                    //content.ShowTextAligned(Element.ALIGN_CENTER, waterMarkName, 0, 0, 0);
                    content.EndText();
                }
            }catch (Exception ex)
            {
                throw ex;
            }
            finally
            {

                if (pdfStamper != null)
                    pdfStamper.Close();

                if (pdfReader != null)
                    pdfReader.Close();
            }
        }
        /// <summary>
        /// 新增傾斜水印
        /// </summary>
        /// <param name="inputfilepath"></param>
        /// <param name="outputfilepath"></param>
        /// <param name="waterMarkName"></param>
        /// <param name="userPassWord"></param>
        /// <param name="ownerPassWord"></param>
        /// <param name="permission"></param>
        public static void setWatermark(string inputfilepath, string outputfilepath, string waterMarkName, string userPassWord, string ownerPassWord, int permission)
        {
            PdfReader pdfReader = null;
            PdfStamper pdfStamper = null;
            try
            {
                pdfReader = new PdfReader(inputfilepath);
                pdfStamper = new PdfStamper(pdfReader, new FileStream(outputfilepath, FileMode.Create));
                // 設定密碼   
                //pdfStamper.SetEncryption(false,userPassWord, ownerPassWord, permission); 

                int total = pdfReader.NumberOfPages + 1;
                PdfContentByte content;
                BaseFont font = BaseFont.CreateFont(@"C:\WINDOWS\Fonts\SIMFANG.TTF", BaseFont.IDENTITY_H, BaseFont.EMBEDDED);
                PdfGState gs = new PdfGState();
                gs.FillOpacity = 0.2f;//透明度

                int j = waterMarkName.Length;
                char c;
                int rise = 0;
                for (int i = 1; i < total; i++)
                {
                    rise = 500;
                    content = pdfStamper.GetOverContent(i);//在內容上方加水印
                    //content = pdfStamper.GetUnderContent(i);//在內容下方加水印
                    
                    content.BeginText();
                    content.SetColorFill(BaseColor.DARK_GRAY);
                    content.SetFontAndSize(font, 50);
                    // 設定水印文字字型傾斜 開始 
                    if (j >= 15)
                    {
                        content.SetTextMatrix(200, 120);
                        for (int k = 0; k < j; k++)
                        {
                            content.SetTextRise(rise);
                            c = waterMarkName[k];
                            content.ShowText(c + "");
                            rise -= 20;
                        }
                    }
                    else
                    {
                        content.SetTextMatrix(180, 100);
                        for (int k = 0; k < j; k++)
                        {
                            content.SetTextRise(rise);
                            c = waterMarkName[k];
                            content.ShowText(c + "");
                            rise -= 18;
                        }
                    }
                    // 字型設定結束 
                    content.EndText();
                    // 畫一個圓 
                    //content.Ellipse(250, 450, 350, 550);
                    //content.SetLineWidth(1f);
                    //content.Stroke(); 
                } 

            }
            catch (Exception ex)
            {
                throw ex;
            }
            finally
            {

                if (pdfStamper != null)
                    pdfStamper.Close();

                if (pdfReader != null)
                    pdfReader.Close();
            }
        }

    }
 

string path = "E://3.pdf";
                HtmlToPDFUtil.HtmlToPdf(Request.Url.AbsoluteUri, path);
                //HtmlToPDFUtil.HtmlToPdf(sbUrlList.ToString(), path);
                PDFSetWaterMark.PDFWatermark(path, "E://3-1.pdf", Server.MapPath("/HtmlToPdf/Tools/sy.bmp"), 0, 0);
                PDFSetWaterMark.setWatermark("E://3-1.pdf", "E://3-2.pdf", "ssss");
                //PDFSetWaterMark.setWatermark("E://3-2.pdf", "E://3-3.pdf", "sssss", "", "", 1);