First you need to define your XMP
tag (a complete example
here)
$ cat config.cfg
%Image::ExifTool::UserDefined = (
'Image::ExifTool::XMP::pdfx' => {
PdfSubTitle => {
Writable => 'string',
},
},
);
1; # end
Then with following command the tag and it’s value will be added:
$ exiftool -config config.cfg -PdfSubTitle="Sub Title" test.pdf
Confirm:
$ exiftool -PdfSubTitle test.pdf
Pdf Sub Title : Sub Title
$ exiftool exiftool test.pdf | grep 'Pdf Sub Title'
Pdf Sub Title : Sub Title