OnionShare isn't a cloud service, it's peer-to-peer over the Tor Network
OnionShare is an anonymous, file transfer program that only uses the Darknet to transfer files. It's not encrypted, but the Onion url is random and isn't going to be guessed, so only the person you send the URL to knows the url exists and must use the TOR Browser to download the file. One person or multiple people may download the same file at the same time.
If the file needs to be encrypted, use PGP to encrypt the file before sending.
OnionShare is available for Mac, Windows and Linux. It comes pre-installed in TAILS, but is easy to install in other platforms. Download instructions for OnionShare are available at Onionshare.org.
Transfering large files over the TOR is going to be slow. It's the way the Onion network is designed to be anonymous by going through multiple nodes. You are able to set and forget the download, but OnionShare and the computer need to remain on.
If you're not already on the TOR network, start up the TOR browser first. Then start Onionshare, drag and drop a file and click Start Sharing. A link for the onion network is generated, copy the link and send to who you want to receive the file.
The person you sent the link to, pastes the link into the Tor Browser and the download starts after clicking on the zip file name and choosing the download directory. The Tor Browser is available at Torproject.org