Note: All Excel protection is relatively weak. It is useful to stop casual users but it is not for top secret information.
It is common for users to send summarised reports in the format of Pivot Tables where confidential information is hidden in the source data, but it is assumed that it is not visible in the Pivot Table. However, even if you created the pivot table from information in another workbook, Excel will remember the data source. So if you want to make sure that no one can see the raw data in a Pivot Table you need to follow some steps.
To follow along go to 5.1