Undo is stored in memiry. There is no possibility now to save/load it with file. If you reload file, you will loose it. Loading undo buffers means you need some mechanism to check if file wasnt modified externally, cause undos must much to exact copy of file. Ifvanything will modify your file from outside, stored undo is useless.
Better is possibility to make a backups and return in time -- <https://forum.pspad.com/read.php?2,74331,74332> PSPad freeware editor https://www.pspad.com
