How to add watermark to PDF in node.js (code example)

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In the project, node is used as the middle layer. During this period, there was a need to add watermarks to PDF files. I searched many posts on the Internet and found the following problems.

1. For a single-page PDF, you can use PNG images to add to the PDF, but the actual usage is not so ideal. It must meet the effect of adding a watermark to each page of the multi-page PDF.
2. I thought about converting the PDF to pictures, adding watermarks to the pictures and then converting them back to PDF, but I felt that the intermediate steps were too cumbersome and the clarity might be reduced.
3. Python and Java have related solutions that can be implemented. I have thought about using node to execute Python scripts to add watermarks to PDFs. Considering that developers are all front-end, if maintenance is needed in the future, expansion may not be so friendly. .

Solution

I browsed Github and NPM and found that node-pdftk can meet the needs, but it has to rely on the PDFTK software. After learning more, I found that PDFTK supports linux. Good , just start using this.

Environment

If docker is used for project deployment, then use docker, which is more suitable for the situation and less troublesome; if you usually develop on your own computer, but If you need a Linux environment to practice or simulate a server environment, you can also use a Linux image to run a dokcer container, which I won’t go into details here.

Implementation

1. app.js

The API is relatively simple. In the input method, we input the source file, and in the stamp method, we input the watermark file. , specify the saved path in the out method. Here we set up a service to generate our watermarked PDF when accessing port 3100.

'use strict';

const express = require('express');
const pdftk = require('node-pdftk');
const path = require('path');
const app = express();

app.get('/', (req, res) => {
    pdftk
        .input(path.resolve(__dirname + '/../files/javascript.pdf'))
        .stamp(path.resolve(__dirname + '/../files/watermark.pdf'))
        .output(path.resolve(__dirname + '/../files/javascript_watermark.pdf'))
        .then(buffer => {
            return res.send('successful');
        })
        .catch(err => {
            console.error(err);
            return res.send('failed');
        });
});

app.listen(3100);

2, dockerfile

FROM node:7.6.0

# 创建工作目录
RUN mkdir -p /usr/src/app
WORKDIR /usr/src/app

COPY ./node_modules /usr/src/app/node_modules

RUN npm set registry https://registry.npm.taobao.org/

# 下载安装PDFTK
RUN apt-get update
RUN apt-get install -y pdftk

# 使用PM2启动应用
CMD ["./node_modules/pm2/bin/pm2-docker", "process.yml"]

3, effect

Source file

Watermark file

After adding watermark

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