I found PDF SAM basic ("split and merge") well done: https://pdfsam.org/en/pdfsam-basic/ https://pdfsam.org/en/pdfsam-basic/. That one is open-source and multi-

I found PDF SAM basic ("split and merge") well done: https://pdfsam.org/en/pdfsam-basic/ https://pdfsam.org/en/pdfsam-basic/. That one is open-source and multi-platform, they have more features in a paying superset project.