DFSG, Licensing & New Packages Team

debian/copyright states that the project was licensed under Apache 2.0 with an exception to allow linking to GPL code.

This statement can be found in the upstream sources in the file
NOTICE. That's probably okay, but not very obvious to see.

Upstream does make another clarification in README.md:

Legal Stuff
PDFio is Copyright © 2021-2025 by Michael R Sweet.
This software is licensed under the Apache License Version 2.0 with an
(optional) exception to allow linking against GPL2/LGPL2 software. See the
files "LICENSE" and "NOTICE" for more information.
The files in pdfio.xcodeproj/* are probably under the same license as the rest,
but again, not exactly obvious. I would drop those files because they are not
needed to build and ask upstream to clarify licensing terms