>Read the registry key at: \\HKEY_Classes_root\.xfdf : finds sub-key AcroExch.XFDFDoc

A correction: easier (and I think, more correct) than using the sub-key, use the default value there:
Read the registry key at: \\HKEY_Classes_root\.xfdf : finds default value "AcroExch.XFDFDoc"