To protect a file or folder in Windows 10:
1. Browse to the folder or file.
2. Right-click, Properties, Advanced
3. Encrypt contents to secure data
A good way to encrypt files across any operating system, and to reasonably protect them from being used even after emailing or copying to another location, is with Microsoft Office. Passwords may be used for individual files.
There are also about a gabillion after-market apps to protect files. I reccomend using the highest encryption your patience / PC can handle. Here are some links - use at your own risk as I cannot attest to any of the software posted here.