Q. How do I use Google to find documents published in specific file formats e.g. PDF or PPT?

Google allows you to refine your search by file format (e.g. pdf, ppt, xls, doc)

Try these search strings:

cloning filetype:pdf

"global warming" filetype:doc

"corporate social responsibility" filetype:ppt

You will notice your results will be in in the format indicated in the search. You can find useful reports, government papers and policies etc. this way. 

Also try Google's Advanced Search page.

Do read the fine print or explore the Help pages.