can I use a fdf file to digitally sign a file

I've got a request to sign a pdf file with an fdf file using .net and I cannot find any information on how to start or even if it's possible.

To my understanding a fdf file, it's just a collection of values not a certificate like also explained here https://docs.appligent.com/fdfmerge/fdfmerge-form-data-format/

An FDF file is a plain text file that contains a list of form fields and their values. Although we’ll show you how to create one by hand, most often FDF files are software generated. These are some of the more common methods to create FDF files:

Use Adobe Acrobat to export forms data directly to a file.

I can use a pfx file to sign a pdf but I've never heard about a fdf file

I would need some help on this topic