新增依賴
<dependency>
    <groupId>com.itextpdf</groupId>
    <artifactId>itextpdf</artifactId>
    <version>5.5.11</version>
</dependency>

String DEST = "D:\\證書.pdf";
//建立com.lowagie.text.Document物件的例項
Document document = new Document();
 PdfWriter writer = null;
 try {
 	 //建立一個書寫器(Writer)與document物件關聯，通過書寫器(Writer)可以將文件寫入到磁碟中
     writer = PdfWriter.getInstance(document, new FileOutputStream(DEST));
     //開啟文件
     document.open();
     //向文件中新增內容
     document.add(new Paragraph("hello"));
     document.add(new Paragraph("world"));
 } catch (Exception e) {
     e.printStackTrace();
 }
 //關閉文件
 document.close();
 writer.close();