Save As .WWF - The PDF you can't print out
saveaswwf.com
saveaswwf.com
In this day and age, I don't care how well intentioned you are, you need to publish a specification for your document format BEFORE you even release a viewer.
And once you publish the specs for your document format, it better have a very good technical reason to exist; inability to print is not one of them.
Now that you have a good reason, make sure your document format is XML-based, or is a superset of another public, and well known one (tool reuse.)
Having done that. Release libraries for processing your format, preferably in MIT or BSD license, and send it up stream to productivity software developers, or offer conversion tools.
Right now these guys fail on all counts. Horrible idea. No spec. Requires binary installation of viewer. Viewer is Mac only. And .. it's raison d'etre is to restrict users and play prohibitionist in their own damn computers, instead of educating them.
The file they have for download is close enough to a standard PDF that the standard Ubuntu PDF document viewer will happily open it and allow you to print. So nothing to worry about here in terms of accessibility in Linux, though I'm guessing it'll annoy Windows users who'll have to rename file extensions.