Musink is a music-score writing program developed by somebody who found the interfaces of other music programs difficult to use while he had a condition making it difficult to use his hands on keyboards or mice.
It is distributed free of charge (in the "lite" but functional version), with a "pro" version with more features available for purchase. It is not "free software" in the sense of open-source projects, however, since the source code is not available.
In addition to its native "Musink project" format (.musink), it is able to export its musical scores in XPS, PDF, and PNG format (the PNG format is output as a separate graphic for each page of the score including a title page at the front, contained in a ZIP file), as well as a playable MIDI file. The music can also be played within the program.
Musink project files are stored in a binary format that does not appear to be documented.
The project files appear to start with the hex bytes
04 00 00 00 0A.