PSPad undo history after file closed

Hi,

First of all, PSPad is an awesome editor!

How do I keep track of the file's undo history even after the file is closed?
Or is this possible in PSPad? Cheers.

When File is closed, no history is kept.
PSPad keeps history even you save file up to time you close file.

For your explanation. It's hard to do it generally.
There isn't problem to save UNDO stack into some form of binary file. Now imagine:
1. you edited and closed your file. PSpad will save undo stack.
2. your file will change in other application
3. you will open file back in PSPad. You have undo stack, but your file content is different. Undo stack is useless, cause it will do everything else than you await from it.