1、引入第三方包

<dependency>
    <groupId>com.itextpdf</groupId>
    <artifactId>itextpdf</artifactId>
    <version>5.5.13</version>
</dependency>
<dependency>
    <groupId>com.itextpdf</groupId>
    <artifactId>itext-asian</artifactId>
    <version>5.2.0</version>
</dependency>

2、水印帮助方法

package com.example.pdf.demo.util;

import com.itextpdf.text.DocumentException;
import com.itextpdf.text.Element;
import com.itextpdf.text.Rectangle;
import com.itextpdf.text.pdf.*;

import javax.swing.*;
import java.awt.*;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;

/**
 * @author tiankk
 */
public class PdfUtils {

    /**
     * pdf添加水印
     * @param inputFile 需要添加水印的文件
     * @param outputFile 添加完水印的文件存放路径
     * @param waterMarkName 需要添加的水印文字
     * @param opacity 水印字体透明度
     * @param fontsize 水印字体大小
     * @param angle 水印倾斜角度（0-360）
     * @param heightdensity 数值越大每页竖向水印越少
     * @param widthdensity 数值越大每页横向水印越少
     * @param cover 是否覆盖
     * @return
     */
    public static boolean addwaterMark(String inputFile, String outputFile, String waterMarkName,
                                       float opacity, int fontsize, int angle, int heightdensity, int widthdensity,boolean cover) {
        if (!cover){
            File file=new File(outputFile);
            if (file.exists()){
                return true;
            }
        }
        File file=new File(inputFile);
        if (!file.exists()){
            return false;
        }

        PdfReader reader = null;
        PdfStamper stamper = null;
        try {
            int interval = -5;
            reader = new PdfReader(inputFile);
            stamper = new PdfStamper(reader, new FileOutputStream(outputFile));
            BaseFont base = BaseFont.createFont("STSong-Light", "UniGB-UCS2-H", BaseFont.EMBEDDED);
            Rectangle pageRect = null;
            PdfGState gs = new PdfGState();
            //这里是透明度设置
            gs.setFillOpacity(opacity);
            //这里是条纹不透明度
            gs.setStrokeOpacity(0.2f);
            int total = reader.getNumberOfPages() + 1;
            System.out.println("Pdf页数：" + reader.getNumberOfPages());
            JLabel label = new JLabel();
            FontMetrics metrics;
            int textH = 0;
            int textW = 0;
            label.setText(waterMarkName);
            metrics = label.getFontMetrics(label.getFont());
            //字符串的高,   只和字体有关
            textH = metrics.getHeight();
            //字符串的宽
            textW = metrics.stringWidth(label.getText());
            PdfContentByte under;
            //这个循环是确保每一张PDF都加上水印
            for (int i = 1; i < total; i++) {
                pageRect = reader.getPageSizeWithRotation(i);
                under = stamper.getOverContent(i);  //在内容上方添加水印
                //under = stamper.getUnderContent(i);  //在内容下方添加水印
                under.saveState();
                under.setGState(gs);
                under.beginText();
                //under.setColorFill(BaseColor.PINK);  //添加文字颜色  不能动态改变 放弃使用
                under.setFontAndSize(base, fontsize); //这里是水印字体大小
                for (int height = textH; height < pageRect.getHeight() * 2; height = height + textH * heightdensity) {
                    for (int width = textW; width < pageRect.getWidth() * 1.5 + textW; width = width + textW * widthdensity) {
                        // rotation:倾斜角度
                        under.showTextAligned(Element.ALIGN_LEFT, waterMarkName, width - textW, height - textH, angle);
                    }
                }
                //添加水印文字
                under.endText();
            }
            System.out.println("添加水印成功！");
            return true;
        } catch (IOException e) {
            System.out.println("添加水印失败！错误信息为: " + e);
            e.printStackTrace();
            return false;
        } catch (DocumentException e) {
            System.out.println("添加水印失败！错误信息为: " + e);
            e.printStackTrace();
            return false;
        } finally {
            //关闭流
            if (stamper != null) {
                try {
                    stamper.close();
                } catch (DocumentException e) {
                    e.printStackTrace();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if (reader != null) {
                reader.close();
            }
        }
    }
}

3、调用帮助方法并输出文件流

package com.example.pdf.demo.controller;

import com.example.pdf.demo.util.PdfUtils;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

import javax.servlet.http.HttpServletResponse;
import java.io.*;
import java.util.UUID;

/**
 * @author tiankk
 */
@RestController
@RequestMapping("file")
public class FileController {

    @GetMapping("downloadFile/{fileName}")
    public void downloadFile(@PathVariable("fileName") String fileName, HttpServletResponse response) {
        //需要添加水印的文件
        String inputFile = "E:\\ChromeDownLoad\\test.pdf";
        //添加完水印的文件存放路径
        String outputFile = "E:\\ChromeDownLoad\\test(水印).pdf";
        //需要添加的水印文字
        String waterMarkName = "测试水印";
        //水印字体透明度
        float opacity = 0.3f;
        //水印字体大小
        int fontsize = 30;
        //水印倾斜角度（0-360）
        int angle = 30;
        //数值越大每页竖向水印越少
        int heightdensity = 20;
        //数值越大每页横向水印越少
        int widthdensity = 4;

        PdfUtils.addwaterMark(inputFile, outputFile, waterMarkName, opacity, fontsize, angle, heightdensity, widthdensity,false);

        fileName = UUID.randomUUID().toString() + "_" + fileName + ".pdf";
        String path = null;
        response.setHeader("content-type", "application/octet-stream");
        response.setContentType("application/octet-stream");
        try {
            response.setHeader("Content-Disposition", "attachment;filename=" + java.net.URLEncoder.encode(fileName, "UTF-8"));
        } catch (UnsupportedEncodingException e2) {
            e2.printStackTrace();
        }
        byte[] buff = new byte[1024];
        BufferedInputStream bis = null;
        OutputStream os = null;
        try {
            path = "E:\\ChromeDownLoad";
            os = response.getOutputStream();
            bis = new BufferedInputStream(new FileInputStream(new File(path + "\\少有人走的路[派克](水印).pdf")));
            int i = bis.read(buff);
            while (i != -1) {
                os.write(buff, 0, buff.length);
                os.flush();
                i = bis.read(buff);
            }
        } catch (FileNotFoundException e1) {
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (bis != null) {
                try {
                    bis.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}