How to create new exif tag for pdf files as using exiftool config file on GNU/Linux?

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