Your notes from iPhone, and iPad automatically sync back to your Mac using iCloud Drive. Just make sure you sign in using the same iCloud account and iCloud Drive for Letterspace is turning on in System Preferences.
You can find all your notes in here:
Finder → iCloud Drive → Letterspace → Inbox Finder → iCloud Drive → Letterspace → Archive
While Letterspace create a new note in .txt format, you can put any text files using any extension as you see fit.
Letterspace also sync with Windows PC (Windows 7 or 8 required). Thanks to iCloud for Windows.
Set up iCloud for Windows PC