pdf's are size agnostic. Unfortunately, most people think that pdf is a synonym for letter size or A4. It's not. You could easily have a print to pdf setting limited to the size of say, a 6" smartphone.
Sorry I wasn't being clear. I meant reading PDFs already created by someone else with a certain paper size in mind, like a scientific paper. Basically, I am looking for something like a "reader view" mode for PDFs on smartphones.