Can the file search extender search within .docx (or .xlsx or .pptx) files?
I originally posted a 'yes' response to your question but I forgot about the fact that the file format specification includes compression so the answer should be 'no'. They would need to be decompressed before you could search them. Your best approach might be to use the COM Automation objects for the associated application to load the document and then use the search methods provided by the COM Automation programming model.