PDF Database is a web search engine for document formats. It concentrates on pdf and doc formats but you will find other formats in the database as well. The developers state that they have 7.5 million documents indexed at the moment. The main interface consists of a search form on top that can search for all document types or only for pdf or doc documents.

Below that is a search cloud that displays the last 200 searches that have been entered in the search form. The results page will display all documents that match the search. The format, name, size and url will be shown. Results can be sorted by size or name.

All tags that have been used to define the document are listed below and will load a new results page when clicked on. All documents found can be downloaded or viewed directly. The view option seems to only work if Adobe Reader and its web plugin have been installed.