To ensure a file is non-recoverable, you will need to permanently delete it from the computer, which can be either it's own hard drive, or a USB memory stick.

When you normally delete a file on your computer, it moves it to the Recycle Bin. This enables you to recover the file if it was deleted by mistake. However, if someone has gained access to your computer, they could recover this file and access it. If the file contains personal identifiable information, then the file should be permanently deleted, which means it doesn't go into the Recycle Bin and it can't be recovered from the computer.

Locate the file that you need to delete and click on it once to highlight it.

If you want to delete more than one file, hold the CTRL key on the keyboard and click on the other files.

Whilst holding the SHIFT key on the keyboard, right mouse/trackpad click the file: or one of the files; you want to permanently delete and select 'Delete'

When prompted, click 'Yes' to confirm you want to permanently delete the file.