https://nextcloud.com/blog/eu-governments-choose-independence-from-us-cloud-providers-with-nextcloud/

This is the successor of the former ownCloud and I have it in use since 2015. The self hosted cloud makes you completely independent from the big players having an eye to your personal data. Even my son uses it 24/7 on his Raspberry Pi 3B+ with an 1TB 2.5" WD HDD for his school magazine compilation and editing, class photo exchange and student research projects. As like dropbox, you can grant access to a shared file for anybody via a link (with or without a timeline). It's available for most platforms including Docker image. For security, the storage media can be encrypted. After the installation there are many collaborative apps available for functional enhancements.

Here's a simple guide for non-techies for a Raspberry Pi server installation https://docs.nextcloudpi.com/en/ which is quite comfortable in comparison what I had to accomplish in 2015 with no ready made RPi image available. The functions are the same as on big data center installations except the performance in case of too many users or high bandwith demand.

Feel free to test it with cheap RPi 3B, a SD card and an USB HDD laying around in the home lab or install it on a VM. :-)