Imrove Searching in Google.com

While you are searching in google some time you face some difficulties. e.g you want to search specific domain sites i.e .com/.net or you want that google show me only pdf/word files. Simply type word that you want to search let say "Windows Wallpaper" just increase one thing in your search is "Windows Wallpaper:net " google reads these words and show you all site of domain .net only.
if you type "learn java:pdf " google show you only pdf files where you can easily download/read online.