PyInstaller is distributed under the GPL license (see the file COPYING.txt in the source code), with a special exception which allows to use PyInstaller to build and distribute non-free programs (including commercial ones). In other words, you have no restrictions in using PyInstaller as-is, but any kind of modifications to it will have to comply with the GPL license. See also our FAQ.

PyInstaller development can only continue if users provide reasonable concrete funding. See Funding PyInstaller for how to support PyInstaller.