You could take a different route: Look through the registry to find the full path to the acrobat .EXE that is registered with the .xfdf file, then use createProcess to launch it, giving the .xfdf file as a parameter:

Read the registry key at: \\HKEY_Classes_root\.xfdf : finds sub-key AcroExch.XFDFDoc
Read the registry key at: \\HKEY_Classes_root\AcroExch.XFDFDoc\CLSID : finds default value {B801CA65-A1FC-11D0-85AD-444553540000}
Read the registry key at: \\HKEY_Classes_root\CLSID\{B801CA65-A1FC-11D0-85AD-444553540000}\LocalServer32 : finds default value "C:\Program Files\Adobe\Acrobat 6.0\Reader\AcroRd32.exe"

So, after doing that programmatically, launch "C:\Program Files\Adobe\Acrobat 6.0\Reader\AcroRd32.exe" with the .xFDF file as a parameter.

(I believe this "walk" is exactly what ShellExecute would do to find how to launch the .XFDF, though, of course, shellexecute doesn't give the ability to wait for the process to end)