Document document = new Document();
PdfWriter pdfWriter = PdfWriter.getInstance(document,new FileOutputStream(PDF文件的地址));
//获取PDF的内容字节
PdfContentByte waterMar = pdfWriter.getDirectContent();
//加载文本
waterMar.beginText();
PdfGState gs = new PdfGState();
//设置笔触字体不透明度为 1f
gs.setStrokeOpacity(1f);
waterMar.setGState(gs);
//设置水印颜色为黑色
waterMar.setColorFill(BaseColor.BLACK);
//设置字体
waterMar.setFontAndSize(BaseFont.createFont(字体文件地址, BaseFont.IDENTITY_H, BaseFont.NOT_EMBEDDED), 字体大小);
//水印的位置坐标
waterMar.setTextMatrix(115, 630);
//水印的内容
waterMar.showText(text);
