使用pdfbox 为 PDF增加水印https://www.jylt.cc/#/detail?activityIndex=2&id=bd410851b0a72dad3105f9d50787f914

引入依赖

<dependency>
    <groupId>org.apache.pdfbox</groupId>
    <artifactId>pdfbox</artifactId>
    <version>3.0.1</version>
</dependency>
<!--下面操作图片的时候需要用到该工具类-->
<dependency>
    <groupId>cn.hutool</groupId>
    <artifactId>hutool-core</artifactId>
    <version>5.8.25</version>
</dependency>

具体代码实现

public static void main(String[] args) throws Exception {
        // 读取原始 PDF 文件
        PDDocument document = PDDocument.load(new File("/Users/a58/Downloads/test.pdf"));

        File file = new File("/Users/picture/Downloads/waterMark.png");
        BufferedOutputStream outputStream = FileUtil.getOutputStream(file);
        FileImageOutputStream fileImageOutputStream = new FileImageOutputStream(file);

	// 创建文字图片
	Font font = new Font("宋体", Font.PLAIN, 12);
        Color fontColor = new Color(100, 100, 100, 60);
        ImgUtil.createImage("水印", font, null, fontColor, fileImageOutputStream);
	// 旋转图片，使水印倾斜
        ImgUtil.rotate(file, -20, file);

        // 遍历 PDF 中的所有页面
        for (int i = 0; i < document.getNumberOfPages(); i++) {
            PDPage page = document.getPage(i);
            PDPageContentStream contentStream = new PDPageContentStream(document, page, PDPageContentStream.AppendMode.APPEND, true, true);

	    // PDF页面宽度
            float pageWidth = page.getMediaBox().getWidth();
	    // PDF页面高度
            float pageHeight = page.getMediaBox().getHeight();

	    // 4：每页有4列水印
            int xBegin = (int) (pageWidth / 4);
	    // 8：每页有8行水印
            int yBegin = (int) (pageHeight / 8);
	  
	    // 加载图片
            PDImageXObject pdImageXObject = PDImageXObject.createFromFile(file.getAbsolutePath(), document);
            float width = pdImageXObject.getWidth();
            float height = pdImageXObject.getHeight();

            for (int yIndex = 0; yIndex < 8; yIndex++) {
                for (int xIndex = 0; xIndex < 4; xIndex++) {
                    contentStream.drawImage(pdImageXObject, (xBegin * xIndex) + 5, (yBegin * yIndex), width, height);
                }
                yIndex++;
            }

            contentStream.close();
        }

        // 保存修改后的 PDF 文件
        document.save(new File("/Users/picture/Downloads/testout.pdf"));
        document.close();
        System.out.println("完成");
    }