Oh Dear - Poor MS
I guess this is why they went out there way to make OOXML, and then ignore it.
Presumably - if you dont know what format your documents are, then you cant search them properly - unless you pay MS for the privilege.
So if you want to search your own documents - make sure you dont upgrade to the next version of office. You save on almost every count. MS looses.
Or leave mechanised 19C filing cabinets, move to late middle 20C computing possibilities and drop office software altogether.