Hazel can do this. The trick is to append or prepend it to the right place.
I think as long as you make sure that your
## NEW lane is the very last one in your Kanban note, you can:
- have Hazel watch the appropriate folder
- when a new pdf is added, Hazel appends a line with a link to the PDF to the Kanban note — and, because NEW is the final header, it should show up in that lane.
A bit of a hack, but it should work.