Document Sync is a private tool run by its operator on their own computer. It is not offered to the public.
The tool asks for one Google permission: access to the files it has created itself in the
signed-in person's Google Drive (scope drive.file). It cannot see or change any
other file in that Drive.
It uploads documents from a folder on the operator's computer to that Drive and reads back the list of the files it uploaded. Nothing else is read.
The sign-in token is stored only on the operator's computer. No data is sent to, stored on or processed by any server other than Google Drive itself. Nothing is shared with third parties, sold or used for advertising.
Access can be withdrawn at any time under myaccount.google.com/permissions. The uploaded files stay in the Drive until their owner deletes them.