A plain and simple solution would be to watch the Download folder and open the PDF with preview. You find a macro here.
In case you don’t know: if you Option-click a PDF link in Safari it will directly download the PDF to the Downloads folder, without opening it in the browser. (This works for links that are not set to open in a new browser window.)
PS: If you are worried about cluttering your Downloads folder with PDFs, you can build another macro with a periodic trigger that removes PDFs from that folder once a week or so.