I've read that you need to download the entire bitcoin blockchain before you can set it to bitcoin core pruned mode to reduce space. But what if I don't have 100gb+ free on my hard drive? Is there a "pruned" blockchain file I can just download so I can skip needing to downloading the entire blockchain?
You can safely set prune either on the command line or in you conf file and Bitcoin Core will handle it automatically. Note that for pruning to operate automatically you need to set it to a number greater than 550 in MB. Pruned mode works automatically when the client is started with pruned mode set.