OffVis (the Office Visualisation tool) is a free package that will display the binary structure of DOC, XLS or PPT files (the older formats, not later incarnations like DOCX). If you think that sounds complicated, then you're right - OffVis is aimed very much at the technical user. But keep reading, it has some security and data recovery applications for the rest of us, too.
The analysis begins by opening a document. Click File > Open Data File, choose a DOC, XLS or PPT file, click OK and you'll immediately see a raw binary view of the file. Then choose an appropriate parser (Word for DOC files, Excel for XLS, PowerPoint for PPT) from the Parser list, and click Parse.
In just a second or two the Parsing Results table will then show the low-level details of your document, the various records and structures that it contains. Security experts can browse these and double-click anything of interest to see it highlighted in the raw binary view.
More generally useful, though, is the program's ability to pick out malicious code. If your document contains any of 16 known exploits then you'll be warned. This isn't a substitute for a real antivirus tool - it's not been updated in a while - but can still provide worthwhile information about a file.
And if your document has been corrupted, maybe is no longer fully readable, then there's a chance (though very faint) that OffVis can help.
Create a copy of your document first, then open and parse this in OffVis. Click Tools > Repair and Defragment, then click File > Save Data File As, give it a unique name, and try opening the document in Office again. OffVis will have tried to fix various file structure-related problems, and it's possible that you'll be able to view the document now, or at least more than you did before. But there are no guarantees and it might have made the situation worse, which is why you must use a copy of the original file. Try it, anyway, see how the program works for you.