HTML-Kit Tools has had the ability to find and replace in multiple documents for a long time. To be able to use this feature though, you first had do set up a project through "File > Projects" and add files and folder targets.
Sometimes you simply need to find in all open documents, without having to set anything up. So starting with HTML-Kit Tools update 20131106 you can do just that.
Find in open documents
To find something in all open documents, press Ctrl+F ("Edit > Find" menu option), enter what you'd like to find and click "Find in Open Documents."
Replace in open documents
To replace something in all open documents, press Ctrl+H ("Edit > Replace" menu option), enter your find and replace text or regular expression, and click "Replace in Open Documents."
The Version History feature ("File > Restore > Restore previous version" menu option) is used to save a copy of each document before updating with replaced text, so you can go back to the previous version if you need to.
If you don't see these options in your HTML-Kit, please make sure that you have the latest version ("Help > Check for Updates" menu option).