A file with .xfdf extension is an XML Forms Data Format that is generated with Adobe Acrobat software. Like FDF, XFDF contains form elements description and their values in XML format. This can include the names and values of text fields in the PDF form. XFDF are saved in human-readable format and can be read programatically read using programming languages such as C#.
XFDF File Format - More Information
XFDF files are saved in XML file format that is a universal format used for import and export of data. An XFDF file structure consists of a header, followed by field values, and elements data as shown below.