<dependency>
  <groupId>com.itextpdf</groupId>
  <artifactId>itextpdf</artifactId>
  <version>5.5.6</version>
</dependency>
<!-- itextpdf的亞洲字體支持 -->
<dependency>
  <groupId>com.itextpdf</groupId>
  <artifactId>itext-asian</artifactId>
  <version>5.2.0</version>
</dependency>

public static void main(String[] args) throws Exception {
    // 模板文件路徑
    String templatePath = "template.pdf";
    // 生成的文件路徑
    String targetPath = "target.pdf";
    // 書簽名
    String fieldName = "field";
    // 圖片路徑
    String imagePath = "image.jpg";

    // 讀取模板文件
    InputStream input = new FileInputStream(new File(templatePath));
    PdfReader reader = new PdfReader(input);
    PdfStamper stamper = new PdfStamper(reader, new FileOutputStream(targetPath));
    // 提取pdf中的表單
    AcroFields form = stamper.getAcroFields();
    form.addSubstitutionFont(BaseFont.createFont("STSong-Light","UniGB-UCS2-H", BaseFont.NOT_EMBEDDED));

    // 通過域名獲取所在頁和坐標，左下角為起點
    int pageNo = form.getFieldPositions(fieldName).get(0).page;
    Rectangle signRect = form.getFieldPositions(fieldName).get(0).position;
    float x = signRect.getLeft();
    float y = signRect.getBottom();

    // 讀圖片
    Image image = Image.getInstance(imagePath);
    // 獲取操作的頁面
    PdfContentByte under = stamper.getOverContent(pageNo);
    // 根據(jù)域的大小縮放圖片
    image.scaleToFit(signRect.getWidth(), signRect.getHeight());
    // 添加圖片
    image.setAbsolutePosition(x, y);
    under.addImage(image);

    stamper.close();
    reader.close();
  }