For now, it's only readable on a Mac, which means that WWF files also cannot be printed from a PC. We're envisioning a world of college professors using these to prevent printable study guides, but so as long as hackers also exist in that same universe, those who prefer their documents on paper -- and HP's ink department -- will probably figure out their own road to nirvana. Must to the dismay of Ma Earth, of course.The WWF format is a PDF that cannot be printed out. It's a simple way to avoid unnecessary printing. So here's your chance to save trees and help the environment. Decide for yourself which documents don't need printing out – then simply save them as WWF.