What is a fillable PDF?
Definition
A fillable PDF is a PDF document that contains interactive form fields — text boxes, checkboxes, radio buttons, and signature areas — that a person or program can complete without altering the underlying page.
Fillable vs. printed
An ordinary PDF is a fixed image of a page: to complete it you would print it and write by hand. A fillable PDF adds a layer of real input fields on top of that page, so the values are captured as data and the document can be completed on screen.
How a PDF becomes fillable
Form fields can be authored by hand in a PDF editor, or detected automatically. Automatic detection reads the page, finds where inputs belong, and places interactive fields there. That is how a flat scan or export becomes fillable without manual work.
Why it matters
Fillable PDFs turn paperwork into structured data: the values typed into the fields can be read back, validated, and stored, instead of living as ink on a printout.