Introduce pypdfium as additional PDF controller
MuPDF is AGPL-3 licensed which users might not want. Add pypdfium as PDF controller, which uses Chrome's BSD licensed PDF viewer pdfium.
Optionally depend on the PyPI version called pypdfium-reboot, which has the license information of pdfium included (current pypdfium misses it).
Wheels include the built library for amd64 on Windows, macOS, and Linux. The module is generally not packaged for individual Linux distributions.