Add Watermark & Stamp to PDF (100% Offline)

How to add a watermark to a PDF offline

Why Client-Side Processing matters for Document Security

When dealing with highly sensitive documents—such as legal contracts, intellectual property, or confidential financial records—uploading your files to traditional online PDF converters poses a massive privacy risk. Once your file hits a remote server, you lose control over who sees it, how long it's stored, and if it's used for AI training data.

The Solution: Zero-Knowledge Architecture.
Our PDF Watermark tool is engineered with a strict client-side-only approach. When you drag your PDF into the browser, the underlying JavaScript engine (powered by pdf-lib) parses the document locally and stamps the watermark directly within your device's memory. Premise: Your files cannot be intercepted. Evidence: You can disconnect from Wi-Fi immediately after the page loads and the watermarking process will still work flawlessly. Conclusion: This is the absolute safest way to watermark a PDF without installing desktop software like Adobe Acrobat.

Top Use Cases for Watermarking PDFs

import { PDFDocument, rgb, degrees } from 'pdf-lib';

async function addTextWatermark(pdfBytes, textStr) {
  // Load the PDF
  const pdfDoc = await PDFDocument.load(pdfBytes);
  const pages = pdfDoc.getPages();
  
  // Loop through all pages
  for (const page of pages) {
    const { width, height } = page.getSize();
    
    // Draw the transparent, rotated text
    page.drawText(textStr, {
      x: width / 2 - 150,
      y: height / 2,
      size: 64,
      color: rgb(1, 0, 0), // Red
      opacity: 0.3,        // 30% visible
      rotate: degrees(45), // Diagonal
    });
  }
  
  // Save and return new PDF bytes
  return await pdfDoc.save();
}

Frequently Asked Questions