AcroForm

An AcroForm is PDF's native, built-in form technology, the kind of interactive form that has been part of the format since the late 1990s. The fillable fields you see in a tax return or an application form, text boxes, checkboxes, radio buttons, dropdowns and signature fields, are AcroForm objects defined directly in the PDF's object structure.

Each field is a named entry with a value, an appearance stream that describes how it draws on the page, and optional rules for formatting or validation. Because the fields are part of the standard PDF object model, essentially any compliant reader can display and fill them, which is why AcroForm is the interoperable choice. The data can be exported as FDF or XFDF, or simply kept inside the document once filled.

This is distinct from XFA, Adobe's separate XML-based form system that many readers never supported. When you flatten an AcroForm, the field values are painted permanently onto the page and the interactive layer is removed, which is a common step before sharing or archiving. All of that field manipulation is plain PDF editing, the kind that runs perfectly well on your own machine.