The sourceforge page (Details tab) says this program is licensed under the GPL. This means, in terms of licensing and copyrights, the program is free to use. You need pay no one for the license, and need do nothing else unless you redistribute it. If you redistribute it, you basically have to include the license and give others the same rights.
Adobe holds
some software patents on the PDF format; but apparently licenses them for free. What might happen if they stopped doing that I'm not sure. I'm not sure of the GPL license version, but that would probably affect what could happen.
And, of course, I am not a lawyer, and this is not official legal advice.