How to Fill PDF Forms in the Browser with pdf-lib (No Server Needed)

I recently added a "Fill PDF Form" tool to my free PDF toolkit PdfWord. Everything runs client-side — the PDF never leaves the user's device. Here's how it works, in case you want to build something similar.

Reading form fields

pdf-lib exposes AcroForm fields directly:

import { PDFDocument } from 'pdf-lib';

const pdfDoc = await PDFDocument.load(arrayBuffer);
const form = pdfDoc.getForm();
const fields = form.getFields();

fields.forEach(f => {
  console.log(f.getName(), f.constructor.name);
  // PDFTextField, PDFCheckBox, PDFDropdown, PDFRadioGroup, PDFButton
});

Filling them

const textField = form.getTextField('full_name');
textField.setText('Shahzaib');

const checkBox = form.getCheckBox('agree');
checkBox.check();

const dropdown = form.getDropdown('country');
dropdown.select('Pakistan');

The minification gotcha

In production builds, field.constructor.name becomes something like e or t — minifiers rename classes. My first version used constructor.name checks and silently failed on the live site while working in dev.

Fix: use instanceof checks against the pdf-lib classes instead:

import { PDFTextField, PDFCheckBox, PDFDropdown, PDFRadioGroup } from 'pdf-lib';

if (field instanceof PDFTextField) { /* ... */ }
else if (field instanceof PDFCheckBox) { /* ... */ }
// ... etc

instanceof survives minification. constructor.name doesn't. This one burned me in production — test your built bundle, not just dev.

Flattening (optional)

If you want the filled values burned into the page so they can't be edited:

form.flatten();
const bytes = await pdfDoc.save();

Updating appearances

One more gotcha: after setting values programmatically, call form.updateFieldAppearances() (or the default font embedding pdf-lib does) so the filled text actually renders in viewers. Without it, some PDF readers show blank fields despite the values being set.

Why client-side?

For a privacy-sensitive feature like form filling (think job applications, tax forms, medical paperwork), "your file never leaves your device" isn't just a technical detail — it's the whole selling point. pdf-lib makes it genuinely feasible: parse, fill, flatten, download, all in the browser.

PdfWord is a free, no-signup PDF toolkit — 27 tools, installable as a PWA, works offline. The fill-form tool and this writeup both came out of building it.