Using Windows Explorer To Search For PDF Files

IFilter is designed to help users to index a large number of documents and then quickly find text within these documents. The documents can be files, email attachments or database records.
Windows Indexing Service, MSN Desktop Search, Internet Information Server, SharePoint Portal Server, Windows SharePoint Services (WSS), Site Server, Exchange Server, SQL Server and all other products based on Microsoft Search technology support indexing technology. Hereafter, we use term "search engine" to refer to these products.

An IFilter acts as a plug-in for full-text search engines. A search engine usually works in two steps:
The search engine goes through a designated place, e.g. a file folder or a database, and indexes all documents or newly modified documents, including the various types documents, in the background and creates internal data to store indexing result.

A user specifies some keywords he would like to search and the search engine answers the query immediately by looking up the indexing result and responds to the user with all the documents that contains the keywords.
During Step 1, the search engine itself doesn't understand format of a document. Therefore, it looks in windows registry for an appropriate IFilter and finds the IFilter. IFilter understands the document format. It filters out embedded formatting and extracts text from the document and returns the text back to the search engine.