FPDM Output Not Saving File

[GreatScott64]

Using FPDM and instead of viewing the merge doc I am attempting to auto-save the doc on the server with variations of this syntax:

$pdf -> Output('filename.pdf', \folder\Destination::FILE);

No matter what I try I get an error and it will not save the results in a PDF file on the server.
(if i do not specify the DESTINATION:: i get incorrect output destination, if I include it then I get a class Destination not found error)

Can anyone help me with a variation of this command that will save the merged PDF doc? or perhaps a different method?

Thanks!

[GreatScott64]

I also found a constructor for the FPDM class where there is a var for output, and the second parameter is used for an FPDF file if any, so it would be the third parameter, but have had no luck passing the ouput file name as a parameter or getting the class to simply save the PDF