WWF is a variant of PDF designed to be non-printable to save trees. It was created by the German chapter of the WWF (the wildlife organization of this name, not the wrestling organization that used to use the same initialism until they changed it after litigation with the wildlife group). You download and install their plug-in, which creates a "print driver" to let you "print" your documents to files of the WWF format, which you can send to others to whom you want to get preachy about how they shouldn't be printing them. Of course, disabling the print function also prevents them from using any other drivers like this one that work by letting you "print" to a driver, so you should hope that no recipient of the file will ever need to use such a thing, like to convert the file to another format later for archiving or preservation.
Files are supposed to be openable in any program that can read PDFs, though if they're saved with a .wwf extension those programs probably won't recognize them as openable (at least without the WWF plugin active).