One of the misleading features of Microsoft Office software is that it gives the user the illusion that they are in control of what's visible and what's hidden to readers of the files. One example is copy-pasting from an Excel sheet into a Word or Power Point. If you now double click on the embedded piece you'll see... the Excel file! It is automatically embedded within the Word/Power Point file. A few years ago, after teaching this to MBAs, a student came the following week all excited, telling me how he just detected fraudulent reporting to his company by a contractor. He simply clicked on a pasted Excel chart within the contractor's report written in Word. The embedded Excel file told all the contractor's secrets.
A solution is to "paste special> as picture". But that's only if you know about this!
Another such feature is Excel's "hidden" fields. You can "hide" certain areas on your Excel spreadsheet, but don't be surprised if those areas are not really hidden: Turns out that Barclays Capital just fell in this trap in their proposal of buying the collapsed investment bank Lehman Brothers. This week's article Lehman Excel snafu could cost Barclays dear tells the story of how "a junior law associate at Cleary Gottlieb Steen & Hamilton LLP converted an Excel file into a PDF format document... Some of these details on various trading contracts were marked as hidden because they were not intended to form part of Barclays' proposed deal. However, this "hidden" distinction was ignored during the reformatting process so that Barclays ended up offering to take on an additional 179 contracts as part of its bankruptcy buyout deal".
(1) if you have secrets, don't keep them in Microsoft Office.
(2) if you convert your secrets from Microsoft to something safer (like PDF), check the result of the conversion carefully!