Search engines only index the files that are "public on the Web". By default, files are private, but you can change the visibility from the "Share" box.
Here are some useful queries that let you find public Google Drive/Docs files (you can append some keywords to the queries):
* [site:docs.google.com/document/d] - find text documents
* [site:docs.google.com/presentation/d] - find presentations
* [site:docs.google.com/drawings/d] - find drawings
* [site:docs.google.com/file/d] - find files: images, videos, PDF files, Microsoft Office documents and more (you should click "repeat the search with the omitted results included" since there are many files with similar titles)
* [site:docs.google.com/folder/d] - find folders (collections of files and other folders)
* [site:docs.google.com/open] - find other documents, folders and files (the links redirect to other URLs)
Public spreadsheets and forms can't be indexed by search engines.