You can protect any file with a password by using the software 7-Zip.
7-Zip is available in the Company Portal (See the guide How to install software for more details on this.)
Once you've installed 7-Zip, when you right-click a file, you will see a 7-Zip menu. Under this menu, you'll see an option to 'Add to Archive...'
Click this and a window will appear, with a number of options. To ensure other people can easily open the file, you first need to change Archive Format to "zip". Then, on the right-hand side of the window, there is an option to encrypt the file with a password. Make sure you make a note of this password, as it cannot be retrieved after the fact.
You can safely leave all other options as default.
Click 'OK', and this will create a .zip file next to where you original file is. This is the encrypted file, and you can now send this out.
When someone tries to open the file, they will be able to see the contents, but will not be able to access / extract them without first entering your password.
If you're looking to password-protect a document created in Microsoft Office, please see How to password protect a document.