Google Docs Viewer & Office Web Apps Viewer
Only files under 25 MB can be previewed with the Google Drive viewer.
Google Drive viewer helps you preview over 16 different file types, listed below:
https://docs.google.com/a/[DOMINIO]/viewer?url=[FILE_URL]
https://docs.google.com/a/[DOMINIO]/viewer?url=[FILE_URL]
##Office Web Apps Viewer ('.ppt' '.pptx' '.doc', '.docx', '.xls', '.xlsx')
http://view.officeapps.live.com/op/view.aspx?src=[OFFICE_FILE_URL]
<iframe src='https://view.officeapps.live.com/op/embed.aspx?src=[OFFICE_FILE_URL]' width='px' height='px' frameborder='0'>
</iframe>
Info: http://www.labnol.org/internet/google-docs-viewer-alternative/