This solution resolves the problem both for Synology Drive Client and for ShareSync.
The annoying problem is that folder tree is copied to the remote NAS, and lnk and zip files are copied. Other files are not copied (like docx, xlsx, graphics).
On the remote NAS, the admin should go to Synology Drive Admin console.
Then clisk Settings (left-side menu),
Then clisk the tab User's syncing profiles,
Then select the current profile from the list.
Then click the button Edit just above.
A window entitled "Edit sync profile" pops up.
In the tab "File Filter" check filetypes in the list of the File Filter. All types should be checked.
The maximum file size should be 0, which means no size restriction.
On the second tab, entitled "Applied User", make sure whether the remote user is listed and has a tick. He should have, by default.
Click Apply.
Now syncing tasks work: documents an graphic files are copied to remote NAS.
It took me weeks to find it out. The reason was that the all filetypes on the file filter list were unchecked by default.
Remember that the File filter list on remote NAS (Drive admin console) defines what the admin of the remote NAS allows you to be backuped.
A similar list on your NAS (in ShareSync) or the one in Synology Drive Client (inder Windows) define filetypes which YOU are intending to send.
Good luck,
