依赖

        <dependency>
            <groupId>com.lowagie.text</groupId>
            <artifactId>com.springsource.com.lowagie.text</artifactId>
            <version>2.0.8</version>
        </dependency>

代码

/**
     * 加水印
     *
     * @param pdfOutStream pdf的字节数组输出流
     * @param outputStream 加完水印后的pdf输出流
     * @param waterMark    水印内容
     * @throws IOException
     * @throws DocumentException
     */
    private void addWaterMark(ByteArrayOutputStream pdfOutStream, OutputStream outputStream, String waterMark) throws IOException, DocumentException {
        StringBuilder waterMarkLine = new StringBuilder(waterMark);
        // 将 defaultWaterMarkNumInLine 条水印内容拼成一长串，这样在pdf中就会显示成连续的一行
        int defaultWaterMarkNumInLine = 10;
        for (int i = 0; i < defaultWaterMarkNumInLine; i++) {
            waterMarkLine.append("  ").append(waterMark);
        }
        // 水印以平均距离展示 waterMarkNum  列在pdf上
        int waterMarkNum = 3;
        PdfReader pdfReader = new PdfReader(pdfOutStream.toByteArray());
        BaseFont base = null;
        try {
          // 字体文件可以放在 项目/src/main/resources/ 下
          // 注意 字体文件如果是.ttc的，路径中 “,1” 是必须的，不然会报错，如果是 .ttf 则不需要
            base = BaseFont.createFont("font/simsun.ttc,1",
                    BaseFont.IDENTITY_H, BaseFont.NOT_EMBEDDED);
        } catch (DocumentException | IOException e) {
            log.error("--> pdf加水印前获取字体失败", e);
            throw new RuntimeException("pdf加水印前获取字体失败！");
        }
        PdfStamper pdfStamper = new PdfStamper(pdfReader, outputStream);
        PdfContentByte content = null;
        Rectangle pageRect = null;
        PdfGState gs = new PdfGState();
        try {
            if (base == null) {
                return;
            }
            // 设置透明度为0.4
            gs.setFillOpacity(0.4f);
            gs.setStrokeOpacity(0.4f);
            int toPage = pdfStamper.getReader().getNumberOfPages();
            for (int i = 1; i <= toPage; i++) {

                pageRect = pdfStamper.getReader().getPageSizeWithRotation(i);
                // waterMarkNum  列水印，文档还会被 (waterMarkNum + 1) 等分
                float xStep = pageRect.getWidth() / (waterMarkNum + 1);
                float yStep = pageRect.getHeight() / (waterMarkNum + 1);
                // 获得PDF最顶层
                content = pdfStamper.getOverContent(i);
                content.saveState();
                // set Transparency
                content.setGState(gs);
                content.beginText();
                content.setColorFill(new Color(224, 224, 224));
                content.setFontAndSize(base, 40);
                // 水印文字成45度角倾斜
                for (int i1 = 0; i1 < waterMarkNum; i1++) {
                    // 计算水印X,Y坐标
                    float x = xStep * (i1 + 1);
                    float y = pageRect.getHeight() - (yStep * (i1 + 1));
                    content.showTextAligned(Element.ALIGN_CENTER, waterMarkLine.toString(), x, y, 45);
                }
                content.endText();
            }
        } catch (Exception e) {
            log.error("--> pdf加水印失败", e);
            throw new RuntimeException("pdf加水印失败！");
        } finally {
            pdfStamper.close();
        }
    }

关于字体文件

如果开发者是Windows系统，字体文件可以在 C:\Windows\Fonts 中找到，复制一份出来就好