There is only one file type that uses the .XFDF extension:
Adobe Acrobat XFDF File
XML Forms Data Format (XFDF) file created by Adobe Acrobat, a PDF authoring application; stores descriptions of page form elements and their values, such as the names and values for text fields; used for saving form data that can be imported into a PDF document.
XFDF files are similar to .FDF files, except XFDF files have a newer format that uses XML formatting.
Updated: June 28, 2018