What Are PDF Form Fields? AcroForms Explained

How Form Fields Work in PDF

Form fields in PDF are widget annotations attached to pages. The document also has an AcroForm dictionary in the Catalog that lists all fields, defines the default appearance, and specifies form submission settings. Each field widget has: a field name (/T) used for data export, a field type (/FT: Tx for text, Btn for button/checkbox, Ch for choice), a value (/V) storing the current entry, and an appearance stream (/AP) defining how the filled field looks. When you fill a form, you're updating the /V values and regenerating appearance streams.

Field Types

• Text fields (Tx): single-line or multiline text input; can be set to accept only numbers, dates, or custom patterns via JavaScript validation
• Checkbox (Btn with flags): binary yes/no; export value is the checked value (e.g., "Yes") or "Off"
• Radio button group: a set of Btn fields with the same parent; selecting one deselects others automatically
• Combo box (Ch): dropdown selection from a list of options; optionally editable
• List box (Ch): visible scrollable list, supports single or multiple selection
• Signature field (Sig): placeholder for a digital signature
• Push button (Btn): triggers an action when clicked — submit form, reset form, open URL

Submitting and Exporting Form Data

AcroForms support three submission methods: FDF (Forms Data Format — a compact format containing just the field name-value pairs), XFDF (XML-based FDF — more portable and standards-compliant), and HTML form POST (submits data as URL-encoded key-value pairs to a web server, just like an HTML form). The submit button widget's action specifies the submission URL and format. For offline forms, data can be exported to FDF/XFDF and emailed separately from the PDF.

JavaScript in AcroForms

AcroForms support JavaScript for: field validation (is this a valid email? is the date in range?), calculation (auto-compute total from line items), formatting (display a number as currency), and custom actions (show/hide fields based on other field values). The JavaScript runs in a sandboxed environment within the PDF viewer. Acrobat Reader has a full JavaScript engine; browsers' built-in PDF viewers have limited or no JavaScript support for forms. Complex calculated forms typically require Acrobat Reader for full functionality.