Index Server will catalog, by default, all the documents stored in the virtual paths set up in Internet Information Server. These can be HTML files, documents created in Microsoft Word, Excel or PowerPoint, or plain text files. Index Server even maintains a list of non-document files in the catalog, such as executables (although such files seldom contain meaningful, human-readable information).
You'll find that there are many document types that are not directly supported by Index Server. At the moment, only the Microsoft formats are supported. However, Index Server has a filter interface, which enables third party software vendors to provide the logic to enable Index Server to support their own application file formats. This means that other suppliers can, potentially, allow any file type to be fully cataloged; and hence the types of files Index Server can search is expected to increase.