If you only need to password protect one file, a free method may be enough. The right answer depends on the file type, how often you edit it, whether you share the computer, and whether the file sits on a USB drive, external drive, or office PC.
Many Word, Excel, PowerPoint, and PDF tools can add an open password or editing restriction. This is often the best free or built-in choice when the file format supports it.
This works well when you need to store or send a private file. It is less smooth when the file changes often, because you need to extract, edit, and package it again.
Permissions can stop other Windows accounts from opening or changing a file. They are useful on a properly managed PC, but confusing on shared family computers or small office machines.
EFS can encrypt a file for your Windows account. It requires careful account and certificate recovery, especially before moving files to another PC.
BitLocker is strong for a laptop drive or external drive, but it does not behave like a simple password box around one selected file.