Devonthink Google Drive Hack

Indexing requires files to be accessible through file system calls. Which n turn means that they have to be either local or on a system accessible via a network file system like SMB/CIFS or NFS. Google drive, Onedrive etc do not offer a network file system. So, the files have to be locally available to be indexable.
Conversely, any file that gets “spirited away” from your local disk must be downloaded again to be indexed.