Serna Free is intended for personal use at home, documenting open-source projects, education, and other non-commercial uses
I downloaded the Mac version and checked the included license file. It includes the following sentence in the section concerning Serna Free (emphasis mine):
Upon your acceptance of this EULA, Syntext freely grants you a personal
non-exclusive, non-transferable, limited license to use the Software on your
computer for personal or business purposes.
Assuming it's the same in the Windows version, there should be no problem.
Additionally, it seems Serna Free has been open-sourced as GPL on GitHub, so if you're able to build it (seems not quite straightforward, but who knows?), you can use it for whatever you like.