What is an AcroForm?
Definition
An AcroForm is the PDF specification's native format for interactive form fields — text boxes, checkboxes, radio buttons, dropdowns, and signature fields embedded in the document. A PDF with an AcroForm can be completed on screen and read back as data.
What it stores
An AcroForm holds each field's name, type, position, and value as part of the PDF itself. Because the fields are structured data, software can read what was entered, set values programmatically, and validate input.
AcroForm vs. flat PDF
A flat PDF has no AcroForm; it is just a page image with no fields. Turning a flat PDF into a fillable one means adding an AcroForm: detecting where inputs belong and writing real fields into the document.