The reason for excluding these types of resources is simply that it would be too expensive and too difficult to attempt to extract a textual representation from them.
The operator of a search engine is simply not motivated to expend the additional resources that would be needed to handle such documents. The gain would be fairly small, and search engines are extremely popular even when they are limited to HTML and plain text documents.
The situation is quite different from the point-of-view of the content provider. A site may have significant amounts of its content in non-textual documents, but despite this the content provider may want to have the documents indexed in normal search engines.
In this paper we present several server-side solutions that allow existing indexing software to index the textual representation of non-textual resources.
