Ah yes... it's all coming back to me! Yes printing to PDF is possible on Windows but the microsoft "print to PDF" driver doesn't permit the security to be set. I'm guessing when I did this last I still had Adobe Acrobat installed which installs it's own print driver. I bet that's what I will have used. Of course I don't have that anymore. Time to look for one of the third party options.
I've got another clunky workround using "PDF Mate PDF Converter for Windows" which does permit me to set the protection I need - sort of. I can protect the document from edits while having reading unprotected but I can't enable the document for document readers, as I would normally do. I think I can live with that - most of my public documents are for photographers and you don't get that many visually impared photographers (although there are some).
I hope Affinity add the option in the future - it seem quite a big ommission from the product.