How do I add a password to particular files and folders?

You can password protect a folder which applies the same password to all files and subfolders within that folder.

Here's how! Simply select the folder you want to apply the password to (or select a few folders if you wish). Then right click and click "Settings".

Then type the password in the required field and press "Save". The password will then be applied to all files and folders existing within that folder.