Maven

<dependency>
  <groupId>com.itextpdf</groupId>
  <artifactId>itextpdf</artifactId>
  <version>5.5.13.2</version>
</dependency>
<dependency>
  <groupId>com.itextpdf</groupId>
  <artifactId>itext-asian</artifactId>
  <version>5.2.0</version>
</dependency>

Gradle

implementation 'com.itextpdf:itextpdf:5.5.13.2'
implementation 'com.itextpdf:itext-asian:5.2.0'

步骤：
第一步：制作PDF
利用WPS、office正常制作pdf
第二步：制作PDF模板
使用Adobe Acrobat Pro DC编辑pdf文档
工具选择准备表单工具，在需要填写的内容处添加文本域，设置字体及字体大小颜色

然后保存pdf
第三步：代码实现填充
PS：由于我做的比较复杂，用别人的例子

import com.itextpdf.text.Image;
import com.itextpdf.text.Rectangle;
import com.itextpdf.text.pdf.*;
import org.springframework.core.io.ClassPathResource;
import javax.servlet.http.HttpServletResponse;
import java.io.BufferedOutputStream;
import java.io.ByteArrayOutputStream;
import java.io.OutputStream;
import java.util.Map;
import com.itextpdf.text.Document;

public class PDFUtil {
    public static void genPdf(Map<String,String> map, HttpServletResponse response) {
        try {
            //模板文件相对路径
            ClassPathResource resource = new ClassPathResource("static/student.pdf");
            PdfReader pdfReader = new PdfReader(resource.getInputStream()) ;
            ByteArrayOutputStream baos = new ByteArrayOutputStream();
            PdfStamper stamper = new PdfStamper(pdfReader, baos);
            AcroFields af = stamper.getAcroFields();
            af.setField("fill_1", map.get("name")) ;
            af.setField("fill_2", map.get("sno")) ;
            af.setField("fill_3", map.get("major")) ;
            af.setField("fill_4", map.get("place")) ;

            // 插入图片
            AcroFields.FieldPosition position = af.getFieldPositions("fill_5").get(0) ;
            int pageNo = position.page ;
            Rectangle signRect = position.position;
            float x = signRect.getLeft();
            float y = signRect.getBottom();
            // 读图片
            Image image = Image.getInstance("E:\\CaiJiXiTong\\图标\\11.jpg");
            // 获取操作的页面
            PdfContentByte under = stamper.getOverContent(pageNo);
            // 根据域的大小缩放图片
            image.scaleToFit(signRect.getWidth(), signRect.getHeight());
            // 添加图片
            image.setAbsolutePosition(x, y);
            under.addImage(image);

            // 设置输出pdf
            // 如果为false那么生成的PDF文件还能编辑，一定要设为true
            stamper.setFormFlattening(true);
            stamper.close();
            //响应到客户端
            String downloadFileName = new String((map.get("name") + "个人信息.pdf").getBytes("utf-8"),"iso8859-1");
            response.addHeader("Content-Disposition", "attachment;filename="+downloadFileName);
            OutputStream os = new BufferedOutputStream(response.getOutputStream());

            Document doc = new Document();
            PdfCopy copy = new PdfCopy(doc, os);
            doc.open();
            PdfImportedPage importPage = copy.getImportedPage(new PdfReader(baos.toByteArray()), 1);
            copy.addPage(importPage);
            doc.close();

            response.setContentType("application/pdf");
            os.flush();
            os.close();

        } catch (Exception e) {
            System.out.println("生成PDF错误："+e) ;
        }
    }
}

填充内容

@GetMapping("download")
public void download(HttpServletResponse response) {
    Map<String,String> map = new HashMap<>();
    map.put("major","计算机生命与科学");
    map.put("name","李小");
    map.put("place","北京");
    map.put("sno","001");
    PDFUtil.genPdf(map, response);
}

我的是填充+新增页面绘制证书