Fill a PDF form

How Fill PDF Form works

The file is read with a FileReader, rewritten in memory and offered as a download. This is why large PDFs are limited by your device's RAM rather than by an upload size limit. A 200 MB scan never crosses the network, so the slowest part is your own disk.

That is the whole architecture: static files, one script, no upload endpoint to leak or log. Close the tab and the work is gone.

What a form field really is

A fillable PDF carries an AcroForm dictionary: a list of named fields, each with a type, a value and a rectangle on a page. The rectangle is the only thing that makes a box look clickable — the page itself is just drawing.

That is why a scan of a form cannot be filled here: it has ink where the boxes are, but no field objects behind them. This tool lists the fields it finds, builds one input per field, and writes your answers back into the same field objects, so the next reader sees a properly filled form rather than text floating over a picture.

Flatten now or leave it editable

Flattening bakes the current appearance into the page content and removes the interactive layer. Do it when the form is finished and going back to an office — nobody can change the salary field afterwards.

Leave it off when the document still has to travel: a second signer, a reviewer adding notes, or a system that reads field values rather than pixels. Flattened text is still selectable and searchable, but no longer editable.

Why it runs in the tab

Forms are the worst possible files to hand to a stranger's server: they carry salaries, medical answers, passport numbers and next-of-kin details. With pdf-lib the whole operation happens in memory, and the only copy that exists at the end is the one you download.

There is no upload endpoint on this site at all, which is a stronger guarantee than a privacy policy promising to delete the file after an hour.

Forms this page cannot fill, and what to use instead

Three kinds of PDF look fillable and are not. A flat scan has boxes printed on it but no fields behind them — use the sign PDF tool and place your text by hand. A digital signature field expects a cryptographic signature, not typed letters, so it stays empty here by design.

The third is XFA: a dynamic form format some government and tax portals still export, where the field definitions live in a separate XML stream. pdf-lib reads AcroForm only, so an XFA file opens with zero fields detected. Open it in Adobe Reader, or ask the issuing office for the AcroForm version — most now publish one.

Password-protected and encrypted files

PDFs carry two different protections and they behave differently. An owner password restricts printing or copying but lets the file open — those load here, because the tool asks the parser to ignore the permission flags. A user password encrypts the content itself, and nothing can read the fields without it.

If a file opens in your reader without asking for anything, it will open here. If it asks for a password first, remove the password in the reader you already trust, then come back.

Why fill a form in the browser at all

The most common PDFs people fill are the ones they would least like to upload: visa applications, medical intake forms, tax declarations, rental agreements with ID numbers attached. A server-side filler asks you to trust a stranger's retention policy; this one has nowhere to send the file.

The trade is honest: no cloud storage means no resume link. Fill it, download it, keep it — the tab keeps nothing after you close it.

Good to know

• Works offline once loaded, including on flights and in locked-down networks.
• No file size limit beyond your device's own memory.
• Free to use, with no account and no usage counter.
• Results are produced locally, so they are not logged or shared with anyone.

Questions