Content on a SharePoint Online site is usually available for downloading with the sync client so that users can work on it offline. In some cases, you may want to prevent users from downloading content with the sync client. Let’s say that you have content containing confidential client information on your site, and you don’t want users to be able to syncitems and take files offline on their laptops or other devices. As a site owner, you can help secure data by setting the Offline Client Availability setting to 'No'.
On the site, click Settings > Site Settings.
Under Search, click Search and offline availability.
In the Offline Client Availability section, select 'No'.
To allow users to download content with the sync client, set the Offline Client Availability option to 'Yes'.
Note: This setting does not affect the ability to manually download documents from the folder.