Google have opened “Docs Viewer,” to all webmasters; now you can just enter the URL of a PDF, Powerpoint or TIFF document and Google'll display it directly in your browser with no download required, or embed it in to your website /blog. To simply the process, Google have created a form to generate code for email, IM, HTML tag, and embedded viewers. Box.net user can also use this service to view documents stored within Box - just add the Google Docs Viewer OpenBox Service to your account.

Google Docs Start Guide:

More info: Google Docs Viewer