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需要iTextSharp.dll, 下载地址http://sourceforge.net/projects/itextsharp/

 　　　　public void Test()
        {
            Watermark(@"E:\日常工作\12084347 config.pdf", @"E:\日常工作\12084347 config wm.pdf", @"E:\日常工作\wm.png");
           
        }        

public bool AddWatermark(string inputPath, string outputPath, string watermarkPath, ref string error)
        {
            try
            {
                PdfReader pdfReader = new PdfReader(inputPath);
                int numberOfPages = pdfReader.NumberOfPages;
                FileStream outputStream = new FileStream(outputPath, FileMode.Create);
                PdfStamper pdfStamper = new PdfStamper(pdfReader, outputStream);
                PdfContentByte waterMarkContent;

                iTextSharp.text.Image image = null;
                if (string.IsNullOrEmpty(watermarkPath))
                {
                    Stream s =  GetType().Assembly.GetManifestResourceStream("WatermarkTool.wm.png");
                    image = iTextSharp.text.Image.GetInstance(s);
                }
                else
                {
                    image = iTextSharp.text.Image.GetInstance(watermarkPath);
                }                
                image.SetAbsolutePosition(100, 100);
                for (int i = 1; i <= numberOfPages; i++)
                {
                    waterMarkContent = pdfStamper.GetUnderContent(i);
                    waterMarkContent.AddImage(image);
                }
                pdfStamper.Close();
                pdfReader.Close();
                outputStream.Close();
                return true;
            }
            catch (Exception ex)
            {
                error = ex.StackTrace;
                return false;
            }
        }

 //选择文件夹
        private void textBox2_DoubleClick(object sender, EventArgs e)
        {
            FolderBrowserDialog dialog = new FolderBrowserDialog();
            dialog.Description = label2.Text;
            if (dialog.ShowDialog() == DialogResult.OK)
            {
                textBox2.Text = dialog.SelectedPath;
            }
        }

        //选择文件
        private void textBox3_DoubleClick(object sender, EventArgs e)
        {
            OpenFileDialog fileDialog = new OpenFileDialog();
            fileDialog.Multiselect = true;
            fileDialog.Title = label3.Text;
            fileDialog.Filter = "*.jpg|*.jpg|*.jpeg|*.jpeg|*.bmp|*.bmp|*.gif|*.gif|*.png|*.png|*.Tiff|*.Tiff|*.Wmf|*.Wmf";
            if (fileDialog.ShowDialog() == DialogResult.OK)
            {
                textBox3.Text = fileDialog.FileName;
            }
        }

 //启动线程
        private void button1_Click(object sender, EventArgs e)
        {
            if (Directory.Exists(textBox1.Text) == false )
            {
                MessageBox.Show(label1.Text, "Require input", MessageBoxButtons.OK, MessageBoxIcon.Warning);
                textBox1.Focus();
                return;
            }
            if (Directory.Exists(textBox2.Text) == false)
            {
                MessageBox.Show(label2.Text, "Require input", MessageBoxButtons.OK, MessageBoxIcon.Warning);
                textBox2.Focus();
                return;
            }
            if ( textBox3.Enabled && File.Exists(textBox3.Text) == false)
            {
                MessageBox.Show(label3.Text, "Require input", MessageBoxButtons.OK, MessageBoxIcon.Warning);
                textBox3.Focus();
                return;
            }

            richTextBox1.Clear();
            button1.Enabled = false;
            Thread thread = new Thread(new ThreadStart(this.BatchDo));

            thread.IsBackground = true;
            thread.Start(); 
        }

 public delegate void SetControlValue(string message);
        //在线程中修改控件属性
        public void AppendRTBText(string text)
        {
            if (richTextBox1.InvokeRequired)
            {
                SetControlValue cal = delegate(string s) { richTextBox1.AppendText(s); };
                this.Invoke(cal, text);
            }
            else
            {
                richTextBox1.AppendText(text);
            }
        }