A Forms Data Format (FDF) file is a formatted ASCII file which contains key:value pairs defining the field names and associated values that are used to populate a form. Forms Data Format (FDF) is defined in the international stand for Portable Document Format; ISO-32000. See ISO-32000-1:2008 Section 12.7.7.
Following is an example of a simple FDF file:
%FDF-1.2
1 0 obj
<< /FDF
<< /Fields [
<< /T (field-name-1) /V (value of field-name-1) >>
<< /T (field-name-2) /V (value of field-name-2) >>
]>>
endobj
trailer
<< /Root 1 0 R >>
%%EOF
FDF is associated with Acrobat Forms (AcroForms) and were introduced in PDF Version 1.2.