@PostMapping("/pdf")
    public byte[] pdf() throws IOException {
        // 構(gòu)建響應(yīng)頭
        String fileName = "example.pdf";
        String encodedFileName = URLEncoder.encode(fileName, StandardCharsets.UTF_8);
        HttpHeaders headers = new HttpHeaders();
        headers.setContentType(MediaType.APPLICATION_OCTET_STREAM);
        headers.setContentDispositionFormData("attachment", encodedFileName);

        // 生成PDF文件
        byte[] pdf = pdfService.pdf();
        // PDF文件
        File pdfFile = FileUtil.createTempFile("demo", ".pdf", null, true);
        FileUtil.writeBytes(pdf, pdfFile);
        // 背景圖片
        ClassPathResource resource = new ClassPathResource("template/picture.jpg");
        File pictureFile = FileUtil.createTempFile("picture", ".jpg", null, true);
        FileUtil.writeBytes(resource.getInputStream().readAllBytes(), pictureFile);
        // 添加背景圖片，獲取添加背景后的PDF文件
        byte[] bytes = addBackground1(pdfFile, pictureFile);

        // 返回
        return ResponseEntity.ok()
                .headers(headers)
                .body(bytes).getBody();
    }

    /**
     * 添加背景圖片
     *
     * @param pdfFile     PDF文件
     * @param pictureFile 背景圖片
     * @return 添加背景后的PDF文件
     */
    private byte[] addBackground1(File pdfFile, File pictureFile) {
        // 定義輸出流，存添加完背景的PDF文件
        ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
        // 透明度設(shè)置為0.2
        float transparency = 0.2f;

        // 加載PDF文件
        try (PDDocument document = PDDocument.load(pdfFile)) {
            // 加背景圖片
            PDImageXObject backgroundImage = PDImageXObject.createFromFileByExtension(pictureFile, document);

            // 創(chuàng)建透明度配置
            PDExtendedGraphicsState graphicsState = new PDExtendedGraphicsState();
            graphicsState.setNonStrokingAlphaConstant(transparency);

            // 遍歷每一頁
            for (PDPage page : document.getPages()) {
                // 獲取頁面尺寸（單位：點(diǎn)，1點(diǎn)=1/72英寸）
                float pageWidth = page.getMediaBox().getWidth();
                float pageHeight = page.getMediaBox().getHeight();

                // 創(chuàng)建內(nèi)容流（追加模式，放在最底層）
                try (PDPageContentStream contentStream = new PDPageContentStream(
                        document, page, PDPageContentStream.AppendMode.PREPEND, true)) {
                    // 應(yīng)用透明度配置
                    contentStream.setGraphicsStateParameters(graphicsState);
                    // 繪制背景圖（鋪滿整個(gè)頁面）
                    contentStream.drawImage(backgroundImage, 0, 0, pageWidth, pageHeight);
                }
            }

            // 保存修改后的PDF
            document.save(outputStream);
            // 以字節(jié)數(shù)組的形式返回加完背景的PDF文件
            return outputStream.toByteArray();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return new byte[0];
    }

<dependency>
	<groupId>org.apache.pdfbox</groupId>
	<artifactId>pdfbox</artifactId>
	<version>2.0.27</version>
</dependency>

    /**
     * 添加背景圖片
     *
     * @param pdfFile     PDF文件
     * @param pictureFile 背景圖片
     * @return 添加背景后的PDF文件
     */
    private byte[] addBackground2(File pdfFile, File pictureFile) throws DocumentException, IOException {
        // 定義輸出流，存添加完背景的PDF文件
        ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
        // 透明度設(shè)置為0.2
        float transparency = 0.2f;

        PdfReader reader = null;
        PdfStamper stamper = null;
        try {
            reader = new PdfReader(FileUtil.readBytes(pdfFile));
            stamper = new PdfStamper(reader, outputStream);
            Image backgroundImage = Image.getInstance(FileUtil.readBytes(pictureFile));

            // 創(chuàng)建透明度配置對象
            PdfGState gState = new PdfGState();
            gState.setFillOpacity(transparency);

            int totalPages = reader.getNumberOfPages();
            for (int i = 1; i <= totalPages; i++) {
                // 直接通過PdfReader獲取頁面尺寸
                Rectangle pageSize = reader.getPageSize(i);
                float pageWidth = pageSize.getWidth();
                float pageHeight = pageSize.getHeight();

                backgroundImage.scaleToFit(pageWidth, pageHeight);
                backgroundImage.setAbsolutePosition(0, 0);

                PdfContentByte content = stamper.getUnderContent(i);
                // 應(yīng)用透明度設(shè)置
                content.setGState(gState);
                content.addImage(backgroundImage);
            }
            stamper.close();
            reader.close();
            
            return outputStream.toByteArray();
        } catch (IOException | DocumentException e) {
            e.printStackTrace();
        } finally {
            if (stamper != null) {
                stamper.close();
            }
            if (reader != null) {
                reader.close();
            }
        }
        return new byte[0];
    }

<dependency>
	<groupId>com.itextpdf</groupId>
	<artifactId>itextpdf</artifactId>
	<version>5.5.13.3</version>
</dependency>