|Well, know that both XLS and whatever type derived from that, is a binary file, and when you start doing text searches in binary files, your result will be based on luck, and luck only.|
It was a coincedence already that you've found the text in the XLS already. If you know the other format is a compressed one, don't even bother doing text search in there.
My advice would be to not do text searches in binaries, over which you have no control, which obviously is the case for XLS files (that is, if they are true Excel files).
Your result may be questionnable. It may work, it may not, it may not work tomorrow, but if may also do.