Building a Serverless PDF Merger & Editor using React and pdf-lib (No Backend!)

I merge PDFs about twice a week. Not enough to pay for an Adobe subscription, but enough to be annoyed by online tools.

You know the drill:

Google "merge pdf free".

Upload your private bank statement to a random server.

Get hit with a "Daily Limit Reached" popup just when you need to download the result.

I got tired of this. I realized that modern browsers and WebAssembly are fast enough to handle PDF manipulation client-side. We don't actually need a backend for this anymore.

So, I built Simple VaultPDF.

The Tech Stack
I wanted to keep it lightweight and 100% offline.

Framework: React (Vite)

PDF Engine: pdf-lib (for merging/splitting) & pdf.js (for rendering)

OCR: tesseract.js

State: Just React hooks, kept it simple.

The Code (How it works)
The coolest part is that pdf-lib lets you manipulate binary data directly in the browser memory. Here is the core function that merges files without sending a single byte to a server:

JavaScript

import { PDFDocument } from 'pdf-lib';

const mergeFiles = async (files) => {
  const mergedPdf = await PDFDocument.create();

  for (const file of files) {
    const fileBuffer = await file.arrayBuffer();

    const pdf = await PDFDocument.load(fileBuffer);

    const copiedPages = await mergedPdf.copyPages(pdf, pdf.getPageIndices());

    copiedPages.forEach((page) => mergedPdf.addPage(page));
  }

  const pdfBytes = await mergedPdf.save();
  return pdfBytes;
};

Why Local-First?
Building it as a Chrome Extension instead of a website had huge benefits:

Zero Server Costs: Since I don't process the files, I don't pay for AWS/Cloud functions.

Privacy: Users don't have to trust me with their data, because the data never leaves their machine.

Speed: No upload/download latency.

The Result
It's currently an MVP. It handles merging, reordering, and rotation perfectly. I also added basic OCR because why not.

I just published it to the Chrome Web Store. If you are tired of paywalls or just want to see how pdf-lib performs in a real extension, give it a try.

Link: Simple VaultPDF on Chrome Web Store

Let me know if you find any bugs! I'm still optimizing the memory usage for large files.

Happy coding!