Content on a SharePoint Online site is usually available for downloading so that users can work on it offline. In some cases, you may want to prevent users from downloading content. Let’s say that you have content that contains confidential client information on your site, and you don’t want users to be able to download items 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, set the Offline Client Availability option to Yes.