This isn't my tool but based on what I read on the previous thread about it, it doesn't seem to be open-source. However, some folks recommended this tool which

This isn't my tool but based on what I read on the previous thread about it, it doesn't seem to be open-source. However, some folks recommended this tool which does seem to run locally: https://github.com/torakiki/pdfsam https://github.com/torakiki/pdfsam