|Is ABCDE located in the same place in every file?|
If so, do you know where it is or do you really have to search for it?
If you have to search for it, do you even know which sheet it's on or do you have to search every sheet?
re: "the actual filename"
Does that mean just the filename or does it mean the full path along with the filename?
BTW...you can't write to a closed file, so the code is going to have to open it, change the string, save it and close it. The files don't have to be visible, but opening, saving and closing "several hundreds of files" is going to take some time.