At first glance, “PDF to DjVu GUI” doesn’t sound like the most promising of programs. Convert documents from one of the most commonly supported formats ever, to something most people don’t know at all? If you need to share files with a wide audience then there’s no way that will ever be a good idea.
And yet, DjVu does have its advantages, especially with document size. We tried out the program with 12 PDF files, and while one DjVu version was fractionally larger, most were 25% – 80% smaller than the original, and a couple dropped by around 90%.