Fill PDF Forms

How to use Fill PDF Forms

Key features of Fill PDF Forms

About Fill PDF Forms

Fill PDF Forms reads the document's AcroForm fields with pdf-lib and draws a matching input for each: a text box, a tick box, a select for radio groups and dropdowns, a multi-select for option lists. Your entries are written into the fields when you save, and pdf-lib regenerates the field appearances, so the values show even in readers that do not redraw forms themselves.

It works for a tax or application form you would otherwise print and fill by hand, a supplier form that arrives as a fillable PDF, or a form you need to fill on a device without a desktop PDF app. It fills only fields that exist: a scanned or flattened form has none, and in that case the tool says so and offers to stamp a name, date and note at the top of the first page instead. Use the PDF Editor to write anywhere on such a form.

When to use Fill PDF Forms

A government, tax or visa form is published as a fillable PDF and asks for identity numbers, dates and addresses. Those details are typed into a page that never sends them anywhere, and the filled_ copy is ready to attach to an application.

You filled a supplier or membership form last year and only the address has changed. Upload last year's copy: text fields open showing their current values, so you change the one box and save, rather than starting again.

An event registration or expense claim arrives as a PDF with checkboxes and a dropdown for the department. Tick the boxes, pick the option and save; the result line tells you how many of the fields were set.

Some forms need a different tool. A signature field is listed but cannot be filled here; draw the signature with Sign PDF after filling. A scanned form, or one that was flattened, has no fields, so write on it with the PDF Editor. If the finished form must not be editable by the recipient, this tool does not lock fields; run the filled copy through Flatten PDF, which draws the values into the page and removes the form.

Tips and troubleshooting

• The line under the filename says which mode you are in: "N Interactive Fields Found" lists the fields, while "Standard PDF" means none were found and only the name, date and note stamp is offered.
• Radio groups and dropdowns open on "(leave unchanged)" and do not display the current choice. Pick a value only when you want to change it. An option list shows no current selection either, and the options you select replace whatever was chosen before.
• "Could not set:" followed by field names means those values were refused, most often text longer than the field's maximum length. The other fields are still written and the file is still saved.
• "Failed to fill form" with an encoding error can appear when a text field contains characters outside the Latin set, such as Chinese or Arabic, because the field's appearance is drawn with the standard Helvetica font.
• "This PDF is encrypted" appears when the form has a password or editing restrictions, even one that opens without a password. Remove the protection with Unlock PDF, then fill the unlocked copy.
• In stamp mode the date is written as year-month-day (for example 2026-10-01), whatever order the date box shows it in, on lines reading "Signed by:", "Date:" and "Note:". They start 50 pt from the left and 50 pt below the top of page 1, over whatever is printed there. Leave a box empty to skip that line; with all three empty, no file is made.
• "Remove File" returns you to the upload box without reloading. The output is named filled_ followed by the original filename, and Preview Document opens it in your browser's own PDF viewer.
• Does the form have no fields at all? How to fill out a PDF that is not fillable shows how to type your answers onto it with the PDF Editor.

How Fill PDF Forms works in your browser

After you choose a file it is read into memory with the File API and parsed by pdf-lib, which returns the document's AcroForm and its list of fields. The tool checks each field's class to decide which HTML control to build: a text input for a text field (pre-filled with the field's current text), a tick box for a checkbox (ticked if the field is), a select for a radio group or dropdown built from the field's options, and a multi-select for an option list.

When you click Save & Download Filled PDF, each control's value is written back through pdf-lib's field API (setText, check, uncheck, select), and pdf-lib regenerates the appearance stream of every field as it saves, so the values are drawn into the page rather than left for the reader to render. The resulting bytes are wrapped in a Blob and offered as a download. Everything you type stays in the page; there is no request that carries the form or its values, and reloading the page clears them.

Frequently asked questions