You can use any cloud service to sync your files, including iCloud, Dropbox and Google Drive. It’s as simple as saving your file in the cloud service of your preference and opening it on the other device. It’s not recommended to keep the files open on both devices however, as some syncing issues may occurr.
macOS
Select Save As… and navigate to your iCloud folder. You can save the file anywhere, but it’s recommended to have a specific folder for your screenplays.
iOS
In the startup view, navigate to your iCloud drive and select Create Document to start working on a new file in cloud. You can also open any Fountain files you’ve saved to your iCloud on desktop version.