AcroForm

AcroForm is the native, built-in form system in PDF. When a PDF has fillable fields, text boxes, checkboxes, radio buttons, dropdowns, you fill in without any special software, those are almost always AcroForm fields. It is the original PDF form technology and the one supported pretty much everywhere.

Each field is a real object in the PDF with a name and a value, which is what lets you tab between fields, type into them, and have the data saved right inside the file. Because AcroForm is part of the core PDF spec, these forms open and work consistently across viewers, browsers, and mobile devices.

This is the practical choice for anything you send out to be completed: application forms, surveys, contracts with blanks to fill. The data stays portable and the form behaves the same no matter who opens it.