Thanks for the replies. Just to clarify, changing the scale does not work, on Firefox, Chrome or Edge the result is the same - I can change the number (eg to 140%) but the preview and the printout do not change, as though the PDF is somehow locked to print at that size.

Even if I use the 'print using the system dialog' option, then go through all the hoops to change the scaling there, I just end up with a blank page. None of this happens with other PDFs.

Just to confirm, without using photoshop I can only ever get a printed image that's 12cm x 8.5cm. Is this what everyone else gets? It just seems far too small to use on a parcel, and I don't think I've ever received anything with an AP label that size, usually they are much closer to A5 (21cm x 15cm).