When you create files with most of Apple's apps (and some third-party ones) it automatically saves several versions of that files so you can easily jump back to a previous version. This is great in most cases, but if you're sharing your computer, you might not want people to have access. Six Colours points to a somewhat hidden menu to delete those files.
If you open up a file in an app like Pages or TextEdit, you can jump into the previous version screen by clicking File > Revert To > Browse All Versions. Here, you can browse through previous versions of the file, so find the one you want to delete.
It will look like the menu bar is gone, but if you mouse over it, it will pop up and you can now access the delete menu at File > Revert To > Delete This Version. Now that versions gone forever.
Tip: Removing previous versions of files [Six Colours]