What license does PDF4QT use and what does that allow?

Updated

PDF4QT is released under the LGPLv3 license, and its source code is hosted on GitHub. The project's stated goal is to offer licensing terms more flexible than GPL or AGPL, so the license is meant to suit both open-source enthusiasts and commercial users. The practical answer for most teams: you can use PDF4QT in a commercial product, but if you modify the library itself and distribute it, the LGPL's copyleft obligations for that library apply — keep the library under LGPLv3 terms and make its source (including your modifications) available.

What LGPLv3 means in practice

The LGPL is a "weak copyleft" license. It separates the library from the application that uses it:

  • Using the library as-is in your own application does not force your application to be open-sourced.
  • Modifying the library and distributing the modified version requires you to release those modifications under LGPLv3.
  • Linking your proprietary application against the library is permitted, provided users can relink the application with a modified version of the library (for example, through dynamic linking or by supplying object files).
  • Distributing binaries of the library requires including the license text and access to the corresponding source.

This is the flexibility the project describes: unlike GPL or AGPL, the LGPL does not extend copyleft to your entire application just because it uses the library.

Who this suits

User type What the license allows What to watch
Open-source developer Use, modify, and redistribute under LGPLv3 Keep modifications to the library under LGPLv3
Commercial application developer Link the library into a proprietary product Use dynamic linking or provide relinking ability; ship license and source for the library
Developer building on the C++ library Access the robust C++ library and command-line tool for script integration Changes to the library itself stay LGPLv3
Team evaluating alternatives to GPL/AGPL tools Adopt a copyleft library without opening the whole codebase Confirm your distribution model meets LGPL relinking requirements

Where the license fits the project's goals

The stated goals explain the choice: create a free and open-source PDF editor alternative to commercial editors, offer licensing more flexible than GPL or AGPL, and support both Windows and Linux with modern C++ and multicore performance. The LGPLv3 choice directly serves the second goal — it keeps the library open while letting commercial users build on it.

Checking the exact terms

The authoritative text is the LGPLv3 license itself, and the project's GitHub repository is the place to confirm the current license file and any notices shipped with releases. If your use case involves static linking, embedded devices, or redistribution under a different license, read the LGPLv3 relinking clauses carefully or consult your legal team before shipping.