I don't think there is a way to download them directly to a PC. You'd have to download them to a Kindle device and then connect that to a PC and copy them off that way.
EDIT: I take that back. I couldn't download them from Manage Your Content and Devices, but I went to
https://www.amazon.com/clouddrive and all of the stuff I've sent to Kindle (only a few docs from way back) are found there in a folder called My Send-to-Kindle Docs and can be downloaded there.