Any electronic document that you would not want to see made public should be encrypted. It is far too easy for a criminal to hack into your PC and retrieve your personal documents. The best way to protect your documents is through the use of strong cryptography like GPG. There are a number of tools that rely on GPG and are relatively easy to use.
You can also choose to encrypt the entire folder where you store documents. Just keep in mind that if you have the folder open when an attacker has access to your PC, the attacker will very likely have access to the encrypted folder. Best practice then is to always close up once you’re done using it.