Use pdf-lib to open the existing PDF, draw text on each page, and save a new file. The library runs in Node.js and exposes controls for the watermark’s position, font, size, color, and opacity. The complete example below adds a centered, semi-transparent “CONFIDENTIAL” label to every page without changing the original file.
What you need
- A current Node.js project using ECMAScript modules (the example uses
import). - An input PDF that your application can read.
- The
pdf-libpackage installed from npm.
Create a project and install the dependency:
mkdir pdf-watermark
cd pdf-watermark
npm init -y
npm install pdf-libAdd "type": "module" to package.json, or use the equivalent CommonJS imports for your project. Put the source PDF in the project directory as input.pdf.
Add a watermark to every page
This script follows the documented pdf-lib workflow: read the bytes, load a PDFDocument, obtain the pages, draw text, serialize the document, and write the result.
import { readFile, writeFile } from 'node:fs/promises';
import { PDFDocument, StandardFonts, rgb } from 'pdf-lib';
const input = await readFile('input.pdf');
const pdfDoc = await PDFDocument.load(input);
const font = await pdfDoc.embedFont(StandardFonts.Helvetica);
const text = 'CONFIDENTIAL';
const size = 24;
for (const page of pdfDoc.getPages()) {
const { width, height } = page.getSize();
const textWidth = font.widthOfTextAtSize(text, size);
page.drawText(text, {
x: (width - textWidth) / 2,
y: height / 2,
size,
font,
color: rgb(0.7, 0.1, 0.1),
opacity: 0.35,
});
}
const output = await pdfDoc.save();
await writeFile('watermarked.pdf', output);
console.log('Wrote watermarked.pdf');Save it as watermark.mjs and run:
node watermark.mjsThe original input.pdf remains untouched. watermarked.pdf contains the additional text on every page, including pages with different sizes because the coordinates are calculated from each page’s own dimensions.
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Control position and appearance
Center the label
page.getSize() returns the page width and height in PDF points. font.widthOfTextAtSize() estimates the rendered width, so subtracting that width from the page width and dividing by two centers the text horizontally. The sample uses height / 2 for the vertical coordinate; adjust that value for a footer, header, or another fixed location.
Change text, color, and transparency
Set text to the wording your policy requires. The rgb() values range from 0 to 1; for example, rgb(0, 0, 0) is black and rgb(1, 1, 1) is white. Lower opacity makes the watermark less intrusive. Preview representative pages on screen and on paper because contrast that looks subtle on a monitor can be too faint when printed.
Use a different standard font
pdf-lib exposes standard PDF fonts such as Helvetica, Times Roman, and Courier through StandardFonts. Replace StandardFonts.Helvetica with the standard font that best fits your document.
Support a custom font or non-Latin text
If the watermark contains characters outside the selected standard font’s coverage, embed a font file instead. pdf-lib’s documented custom-font integration requires registering @pdf-lib/fontkit and embedding the font bytes before calling drawText:
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchnpm install pdf-lib @pdf-lib/fontkitimport { readFile } from 'node:fs/promises';
import { PDFDocument } from 'pdf-lib';
import fontkit from '@pdf-lib/fontkit';
const pdfDoc = await PDFDocument.load(await readFile('input.pdf'));
pdfDoc.registerFontkit(fontkit);
const fontBytes = await readFile('fonts/NotoSans-Regular.ttf');
const font = await pdfDoc.embedFont(fontBytes);
// Use this font in page.drawText({ font, ... })Choose a font licensed for your use, and verify the output in the PDF viewers your recipients use. A font that lacks a required glyph can produce missing or incorrect characters.
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Watermark only selected pages
getPages() returns an array, so you can select pages by index. PDF page indexes are zero-based:
const pages = pdfDoc.getPages();
for (const index of [0, 2, 4]) {
const page = pages[index];
if (!page) continue;
page.drawText('DRAFT', {
x: 36,
y: 36,
size: 18,
font,
color: rgb(0.2, 0.2, 0.2),
opacity: 0.4,
});
}For a contiguous range, loop from the first index through the last and check that each index exists. If the source can be empty or shorter than expected, always guard against an undefined page.
Build a reusable watermark function
Putting the operation in a function makes it easier to process uploads, jobs, or multiple files:
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import { PDFDocument, StandardFonts, rgb } from 'pdf-lib';
export async function addWatermark(inputPath, outputPath, label) {
const pdfDoc = await PDFDocument.load(await readFile(inputPath));
const font = await pdfDoc.embedFont(StandardFonts.Helvetica);
const size = 24;
for (const page of pdfDoc.getPages()) {
const { width, height } = page.getSize();
const textWidth = font.widthOfTextAtSize(label, size);
page.drawText(label, {
x: Math.max(0, (width - textWidth) / 2),
y: height / 2,
size,
font,
color: rgb(0.7, 0.1, 0.1),
opacity: 0.35,
});
}
await writeFile(outputPath, await pdfDoc.save());
}
await addWatermark('input.pdf', 'watermarked.pdf', 'CONFIDENTIAL');For untrusted uploads, write to a controlled temporary directory, impose your own file-size and execution limits, and remove temporary files after the job. Keep the output path distinct from the input until validation succeeds.
Diagonal and advanced layouts
A diagonal watermark can be useful, but the exact text-transform options depend on the pdf-lib version you install. The documentation establishes text drawing and styling, not a single production recipe for every rotated layout. Check the API reference for your installed version before relying on a rotation option, then test pages with portrait and landscape dimensions. If rotation is not available in your version, place the label horizontally or draw it in a pre-rendered asset with a tool that supports the transform you need.
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For repeated labels, headers, or footers, calculate coordinates from width and height rather than hard-coding one page size. Leave a margin so the label does not run into trim or printer margins.
Common failures and fixes
“Cannot use import statement outside a module”
Node is treating the file as CommonJS. Add "type": "module" to package.json, use an .mjs extension, or convert the imports to your project’s CommonJS convention.
The output is blank or the load fails
Confirm that the input path points to a PDF and that the process has read permission. A truncated, malformed, encrypted, or otherwise unusual file may not load successfully. Catch the exception, preserve the original upload, and report the file as unsupported rather than overwriting it.
The watermark is invisible
Increase opacity, choose a contrasting color, increase size, and verify that x and y fall inside the page bounds. Remember that PDF coordinates start at the lower-left in the usual coordinate system, so a small y value is near the bottom.
Characters display as boxes or disappear
The selected font does not contain those glyphs. Embed a suitable font with @pdf-lib/fontkit, and check the font’s license and coverage.
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Only some pages have a watermark
Ensure the drawing call is inside the loop over pdfDoc.getPages(). If you intentionally selected indexes, remember that they start at zero and that a missing index is skipped.
Forms, encryption, or complex PDFs behave differently
The reviewed documentation confirms existing-document modification and text drawing, but it does not establish universal behavior for every encrypted, malformed, form-heavy, or unusually structured PDF. Validate the actual files your service receives and inspect the result in the target viewers before promising compatibility.
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- Embedding one font before the page loop avoids repeating the same embedding work for every page.
- Process large files in a queue or worker when request timeouts are a concern; the complete document is loaded and serialized in memory by this pattern.
- Write to a temporary output, reopen or otherwise validate it in your pipeline, then atomically move it into place so a failed save cannot replace a good file.
- Test a representative set: different page sizes, many-page files, scanned pages, selectable text, custom glyphs, and printed output.
- Keep pdf-lib and your Node.js runtime maintained according to your organization’s dependency policy. No independent performance benchmark or Node.js version matrix is established here.
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Does drawing a watermark make a PDF impossible to edit?
No. This operation adds page content; it is not a security or digital-rights control. Readers may still edit, copy, or remove content with tools that support those operations.
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Can I preserve the original PDF?
Yes. Read the source and write a separate output path, as the examples do. Keep the original until the new file has passed your validation checks.
Is a watermark applied to annotations or attachments?
The documented method draws text on pages. It does not establish behavior for every annotation, attachment, form, or encrypted-document feature, so test those structures in your own files.
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Can I preserve the original PDF?
Yes. Read the source and write a separate output path, then validate the new file before replacing or distributing anything.
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Is a watermark applied to annotations or attachments?
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