Why do some files on iCloud suddenly force me to download again? I found that sometimes my files which I put on my iCloud are going to disappear from my local hard drive and thus force me to download them again. These files have the "download from the cloud" icon on Finder, and I'm not sure when and why these icons are added to which files.

I don't like to download them again, but this happens quite often. The files are put under ~/Library/Mobile Documents/com~apple~CloudDocs/my_directory. How can I avoid them and keep the files available at local?
This is the correct situation if your Mac has Optimize Mac Storage enabled in the iCloud preference pane.



If your storage is under light pressure, I don't find recently used documents being released from local storage much if at all, but if you are running with a very close to full drive, the system may be removing the local copy before you like.



Above is an SSD under light pressure - lots of space for local backups, iCloud data to remain cached and space to add new files. Below is iCloud showing nearly 200 GB of data so if all of that were on my MacBook - my pressure would be constrained / close to full.



There's nothing sudden about this, though in my experience. Most people I assist with this are more bothered by the large amount of data that could be "purged" and isn't but I get your not wanting to re-download and wait for files.