If you are talking about using the Report Generation Toolkit, the report format for Word reports is .doc format, and for Excel spreadsheets is .xls format. These are binary files whose format is proprietary to Microsoft. If you're talking about an HTML report, obviously the generated file is a .html file that is viewable in a standard text editor. And for Standard Reports, there is no file generation (they get sent straight to the printer).
Hope this helps,
-D
