Are you working with sensitive information in Microsoft Word, PowerPoint, or Excel?
A reasonable option is to use the “Encrypt with Password” option that is available in all Office programs in the File menu by selecting Info
When you select this option, you will be prompted to provide a password that will be required to every time you open the document.
Then you will be prompted to confirm the password.
Once you have confirmed the password, your file has been encrypted.
When you go to open the document, you will be prompted for the password.
Should you want additional security on a file you might consider using GPG for Mac to encrypt your confidential files.