PDF are created so they should be device independent so long as the device has a big enough screen to view them as intended.
So to answer your question
Old World New World Used for
PDF reader with appropriate sized screen PDF reader with appropriate sized screen Seeing documents presented in the exact format the publisher intended
Reading PDF's on a 7" tablet is painful but reading them on a 6" Eink with a slower refresh is really painful.
If you need to read PDF's use a device with a larger screen.