Up to ten currently opened files are listed at the bottom of the File Menu for easy access. The eleventh file opened will replace the first file on the list and so on. Select a file from the list to display it.

If you forget that a file is already opened and you try to Open the file again, you will be reminded that the file is already opened and asked if you want to reload the original.  The usual response should be No and ZW3D will reopen the active one.