This article explains what an XFDF file is and how to open one.
How to open more than 2 PDFs with Acrobat?
A file with the XFDF file extension is an XML Forms Data Format file that stores information usable by a PDF file, such as the values in the various forms of the document. The XFDF file inserts that data directly into the PDF.
For example, if multiple forms in a PDF file need to be filled in with a user's data, that data is first extracted from a database containing that data and saved in the XFDF format so that it can be used in the PDF file.
FDF files are similar to XFDF files, but use PDF syntax instead of XML markup.
