Google Docs and Google Drive don't offer the option to find public files, but you can do that using Google search. Now that the Google Docs robots.txt file allows search engines to index most of the public files, you should be able to find them, assuming that there are some links that point to them.
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.