Files may not be as secure as you think if you use only this option.
Some of the settings that appear on the Security tab, including some that sound like security features, do not actually help secure workbooks.
For example, Read-only recommended does not secure a workbook. It is only a guideline for readers — someone could still edit the data.
But if you would like to remind others that data is important and should not be changed, you can have Excel recommend that a workbook be opened as read-only.
When you select the Read-only recommended check box, users will get a read-only recommendation when they open the file. But keep in mind that users can ignore the recommendation and open the file to make changes in it.