How to find and change permissions for files and folders shared from SharePoint
1. Open the site where you want to run the report
2. On the Settings menu, click Site usage. Then scroll down.
3. In the Shared with external users section, click the Ellipse menu for the file you want to change, then select View permissions.
4. Choose the desired option on the pop up menu to Stop sharing completely or for individual users. If the folder that the file is in is shared you will see a notice. You may need to change the folder permissions also.