WWF is a with PDF comparable but non printable document format distributed by World Wide Fund For Nature (WWF).
Because every day, an entire forests are cut down to make paper, WWF invented a not printable file format to save paper.
Instead of printing you are using the option "Save as WWF" to get the new .wwf document from every kind of application. Before doing this, you have to download and install the SAVE_AS_WWF_1.0 software, available for different operating systems. http://www.saveaswwf.com/en/what-is-it.html
Without having the software installed you can open the file with Acrobat Pro or Acrobat Reader.