How to use it
- Choose the PDF form. The number of fields appears under the name, and each field appears as a box below.
- Type in the text boxes, tick the checkboxes, choose one radio button per group and pick the values in the dropdowns. Read-only fields stay as they are.
- Tick Flatten the form only if the answers must not change afterwards. A flattened form is a picture of the answers on the page.
- Click Fill in the form, then Download to save the filled file.
The kinds of field this tool fills
Most PDF forms use AcroForm fields: text boxes, checkboxes, radio button groups, dropdowns and list boxes. The tool reads them with the open-source pdf-lib library, writes your answers into them and saves the file. Fields keep their names, so the form still works in other programs.
- Text boxes and multi-line boxes, with the length limit the form sets.
- Checkboxes, ticked or not.
- Radio button groups, one choice each.
- Dropdowns and list boxes, with the options the form offers.
- Buttons and signature areas are listed as not fillable here; signatures are added with the sign tool.
XFA forms and letters in the answers
Some forms are built with XFA, a format of dynamic forms that a PDF reader must run. This tool does not fill XFA forms, and it says so before it changes anything. Open such a form in a PDF reader.
Flattening draws an answer that holds a letter the standard font lacks, such as Turkish ş or Cyrillic, as a picture of the answer in the field's place, so the flattened page shows it in full. Without flattening, the answer stays in the field, and your PDF reader draws those letters itself when it opens the file.
Example: an application form with a checkbox and a dropdown
A form has a name box, a checkbox for "I agree", a radio group for "Female" or "Male" and a dropdown for the city. You type the name, tick the box, choose Female and pick Ankara, then save without flattening.
The file keeps the answers in its fields, so the recipient can still change them in a PDF reader.
Frequently asked questions
Are my forms uploaded anywhere?
No. The form is read, filled and saved by your browser on your own device. Nothing is sent to our server.
Why does the tool say my form uses XFA?
XFA is a dynamic form format that needs a PDF reader to run it, and this tool does not fill it. The message appears before anything is changed, so your file is safe. Open it in a PDF reader instead.
What is the difference between saving and flattening?
Saving keeps the fields, so the answers can be changed later. Flattening draws the answers into the page and removes the fields, so they can no longer be edited. Flatten only the final version.
Why does a letter look different in a filled form and in a flattened one?
A filled form keeps the answer in its field, and the PDF reader draws the letters the standard font lacks. A flattened form makes each answer part of the page: letters the standard font cannot write are drawn as a picture of the text, so they look the same in every reader.
Why is a field shown as read only?
The form's author locked it. The tool does not change locked fields, so their values stay as the file has them.
Does it work on a phone?
Yes, in any up-to-date browser. Tap the boxes to type, tick the checkboxes and choose options from the lists the form offers.
Sources
- pdf-lib, the open-source library that reads, fills and flattens the form fields in your browser
- pdf-lib documentation: reading and changing existing PDFs, their page boxes, document information and form fields
Figures and text last checked on Tuesday, November 10, 2026.