Open the document you wish to protect, this process is the same for both Word and Excel documents 

 

Go to: File Info

 

 

Then select Protect Document, then Encrypt with a Password

 

 

 Put a password in the box, it will come up again to verify that it is correct, make sure you remember this as without it, your document will not be accessible. 

 

This will then ask the user for the password before allowing them to open it, meaning it is secure.