I have created an A5-sized information sheet using Word 2016 which I save as a PDF (using File > Save As then selecting PDF file type).

That gets me a PDF which opens OK in Adobe Reader and seems to display as an A5 document (148 x 210 mm). However when end users print the PDF it comes out on A4 paper. How do I use Word 2016 to generate a PDF that the end user cannot print on any size of paper except A5? How do I make it so that printers and printer drivers cannot over-ride the paper size that I, as the author, have chosen?

Simply telling people to print it on A5 paper isn't working as they just hit print without checking what paper size appears in the print dialog box.