On Internet there are millions of files with different formats. Google recognizes them and hence you may search web for a specific file format. Likewise if you want to search certain book in PDF format then you should type the book’s name (in double quotes) followed by filetype and then book’s format in Google’s search box as show in the following given example:
Example: If you want to search book After The Software Wars in PDF format on world wide web:
Type as show below in Google’s search box
“After the Software Wars” filetype:PDF