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.
Brod Grety commented
Hi, what do you know about adaptive design ? Users who come to your resource are not the same devices, and from this resource may not function well or function well at all. That would not be the case need to make adaptive design. If you want to know more, I recommend you https://www.halo-lab.com/services/responsive-website-design creating a mobile-friendly website. It says everything from creation to testing and release the product.