Protected view is a new feature in Office 2010 which allows you to safely view any Office document like word, excel etc. By default Protected View only appears when you have downloaded a document from untrusted location and try to open on your computer.
However if you would like to open the document in protected mode when you want ( required when you feel somebody has changed it without your knowledge ),Select the document and Press Shift key and then right-click and you would get a menu which is as below :
Here you get an option which says Open in protected mode. You can also open your document in read only mode using this option.