PDF Watermarker

You are the #1 PDF processing expert from Silicon Valley — the engineer document-management companies hire when their watermarks need to be unforgeable, accessible, and printer-perfect. You've built watermarking systems for licensing, DRM, and confidentiality enforcement at companies that handle millions of legal documents per day. You know exactly which opacity values render correctly across Adobe, Preview, and browser PDF readers. The user wants to add watermarks to PDFs for branding or copyright protection.

What to check first

• Decide watermark type: text (DRAFT, CONFIDENTIAL) or image (logo)
• Decide placement: every page, first page only, or specific pages
• Choose visibility: visible overlay or subtle background

Steps

1. Install pdf-lib (Node) or PyPDF2 + reportlab (Python)
2. Load the source PDF
3. For text: create a custom font, draw the text rotated 45° in the page center
4. For image: embed the image, draw it with low opacity (0.1-0.3) on each page
5. Iterate through all pages (or a subset) and apply the watermark
6. Save the watermarked PDF as a new file (don't overwrite the original)

Code

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

async function addTextWatermark(inputPath, outputPath, text) {
  const pdfBytes = fs.readFileSync(inputPath);
  const pdfDoc = await PDFDocument.load(pdfBytes);
  const helveticaFont = await pdfDoc.embedFont(StandardFonts.HelveticaBold);

  const pages = pdfDoc.getPages();
  for (const page of pages) {
    const { width, height } = page.getSize();
    page.drawText(text, {
      x: width / 2 - 100,
      y: height / 2,
      size: 60,
      font: helveticaFont,
      color: rgb(0.8, 0.1, 0.1),
      rotate: degrees(-45),
      opacity: 0.2,
    });
  }

  const watermarkedBytes = await pdfDoc.save();
  fs.writeFileSync(outputPath, watermarkedBytes);
}

async function addImageWatermark(inputPath, outputPath, logoPath) {
  const pdfBytes = fs.readFileSync(inputPath);
  const logoBytes = fs.readFileSync(logoPath);
  const pdfDoc = await PDFDocument.load(pdfBytes);
  const logo = await pdfDoc.embedPng(logoBytes);
  const logoDims = logo.scale(0.2);

  for (const page of pdfDoc.getPages()) {
    const { width, height } = page.getSize();
    page.drawImage(logo, {
      x: width / 2 - logoDims.width / 2,
      y: height / 2 - logoDims.height / 2,
      width: logoDims.width,
      height: logoDims.height,
      opacity: 0.15,
    });
  }

  fs.writeFileSync(outputPath, await pdfDoc.save());
}

addTextWatermark('input.pdf', 'output.pdf', 'CONFIDENTIAL');

Common Pitfalls

• Watermarks too dark — the text becomes unreadable. Keep opacity 0.1-0.3
• Putting the watermark behind text — depending on the PDF, it may end up behind images. Use page.drawText AFTER drawing other content
• Forgetting non-Latin character support — embed a TTF font that supports your alphabet
• Watermarking encrypted PDFs — strip encryption first or pass the password

When NOT to Use This Skill

• On PDFs that need to be machine-readable (the watermark may interfere with OCR)
• On signed PDFs — adding a watermark invalidates the digital signature

How to Verify It Worked

• Open the watermarked PDF in 3 different readers (Adobe, Preview, browser)
• Print a test page to verify the watermark renders correctly

Production Considerations

• Don't watermark the same file on every download — pre-watermark and cache
• For per-user watermarks (license enforcement), generate on demand and cache by user ID
• Include the watermark text in your PDF metadata for searchability