1.添加依赖

<!-- pdf打水印 -->
<dependency>
    <groupId>com.itextpdf</groupId>
    <artifactId>itextpdf</artifactId>
    <version>5.5.13</version>
</dependency>

2.下载所需要的字体

去找到相关字体的ttf文件，这里使用的是宋体。 可以使用如下连接下载。 「simsun.ttf」www.aliyundrive.com/s/STggfheok… 提取码: 9uw3 点击链接保存，或者复制本段内容，打开「阿里云盘」APP ，无需下载极速在线查看，视频原画倍速播放。

3.添加工具类

package com.hobby.item.util;

import com.itextpdf.text.BaseColor;
import com.itextpdf.text.DocumentException;
import com.itextpdf.text.Element;
import com.itextpdf.text.pdf.BaseFont;
import com.itextpdf.text.pdf.PdfContentByte;
import com.itextpdf.text.pdf.PdfReader;
import com.itextpdf.text.pdf.PdfStamper;
import org.springframework.util.StreamUtils;

import java.io.*;

/**
 * PDF 加水印
 */
public class PdfMarkUtil {

    public static byte[] addWatermark(InputStream inputStream, String watermark) throws IOException, DocumentException {
        PdfReader reader = new PdfReader(inputStream);
        try (ByteArrayOutputStream os = new ByteArrayOutputStream()) {
            PdfStamper stamper = new PdfStamper(reader, os);
            int total = reader.getNumberOfPages() + 1;
            PdfContentByte content;
            // 设置字体
            String fontPath = "C:\Users\Administrator\Downloads\simsun.ttf";
            //设置字体
            BaseFont baseFont = BaseFont.createFont(fontPath, BaseFont.IDENTITY_H, BaseFont.NOT_EMBEDDED);
            // 循环对每页插入水印
            for (int i = 1; i < total; i++) {
                // 水印的起始
                content = stamper.getUnderContent(i);
                // 开始
                content.beginText();
                // 设置颜色
                content.setColorFill(new BaseColor(244, 244, 244));
                // 设置字体及字号
                content.setFontAndSize(baseFont, 50);
                // 设置起始位置
                content.setTextMatrix(400, 780);
                for (int x = 0; x < 5; x++) {
                    for (int y = 0; y < 5; y++) {
                        content.showTextAlignedKerned(Element.ALIGN_CENTER,
                                watermark,
                                (100f + x * 350),
                                (40.0f + y * 150),
                                30);
                    }
                }
                content.endText();
            }
            stamper.close();
            return os.toByteArray();
        } finally {
            reader.close();
        }
    }


    public static void main(String[] args) throws IOException, DocumentException {
        File file = new File("C:\Users\Administrator\Desktop\test.pdf");
        InputStream inputStream = new FileInputStream(file);
        byte[] bytes = addWatermark(inputStream, "哈哈哈水印");
        OutputStream outputStream = new FileOutputStream(new File("C:\Users\Administrator\Desktop\test-mark.pdf"));
        StreamUtils.copy(bytes,outputStream);
        outputStream.close();
    }

}

4.效果图