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?
Pruning saves disk space, but you still need to download the whole blockchain,
prune mode works on the fly, once your node has downloaded the block data and validated the blocks/transactions in it, it throws away the old data that it no longer needs, but in order to revalidate all old blocks
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.