In this article, I will give you a brief overview of the Form types you can find inside a PDF containing AcroForms.
The data for an AcroForm is stored inside standard COS objects in a PDF. You will find most of the AcroForms data linked to the name ‘AcroForm’ Object inside a PDF file. This will then have a ‘Fields’ tag which will then list lots of PDF references which are all AcroForms. Some files will only have one or two, others contain hundreds.
The key value is th ‘FT’ entry which defines the type of Form object. What other values are present will depend on this value. Here are the possible values.
Tx = is a text field.
Ch = is a choice field.
Btn = is a Button field.
Sig = is a Signature field.
Now we know the type of field, but there are multiple fields within most types. To find out which exact field you have you also need the value for tag name ‘Ff’, this is a byte sequence where each byte represents a different flag.
These are :-
Tx = Text (defaults to a single line text field, the flags are options)
byte 13 = multiline
byte 14 = password
Ch = choice
byte 18 = combo
no flag = list field
Btn = button
byte 16 = radio
byte 17 = push button
neither flag = check box button
We have lots more articles about both PDF Forms and PDF files in our series of Articles on Understanding the PDF File format.