there should be a way to set a default paper size for your printer in Acrobat Reader DC
For some reason, Acrobat Reader DC assumes that all .pdf files will need to print on A4 paper. My printer is configured for Letterhead (8.5X11). I can reset the size when printing but should not need to do so. I'm fairly certain that none of the institutions sending me .pdf files are using A4 as they are located in Canada and Canada is still using Imperial sizes to maintain compatibility with the USA.