The filetype operator allows you to search for pages that end with a particular file extension, i.e. restricts a search to a certain file type.
Example: filetype:xls OR filetype:doc OR filetype:pdf site:nus.edu.sg +answers
Note that from the above example, you can view PDF files with Google Docs by clicking on Quick View. Google will also convert documents into HTML for viewing. You can view a document in HTML format by clicking on the View as HTML link.
This is useful when you do not have a required document viewer installed on your computer. However, it is always better to view the documents in their original format as the HTML conversion is not perfect - some of the contents may be missing.
Almost every file extension can be searched for. FILExt has a comprehensive list of file extensions. This operator can effectively narrow your document search when used with the site and / or intitle operators. Note that using a NOT operator with the filetype search term will not produce any results.
Examples that will not show any results:
filetype:txt -inurl:txt
filetype:txt -txt
A related operator is the ext operator.
