Google Docs Viewer allows preview of all kinds of documents such as PDF, PowerPoint, Word and spreadsheet directly in the Browser. You just have to prefix the Google Docs viewer URL in the file template. Instead of

<a href="%file_url%">%file_display_name%</a>

use

<a href="http://docs.google.com/viewer?url=%file_url_encoded%">%file_display_name%</a>

To embed the document use this template code:

<iframe style="border: none;" src="http://docs.google.com/viewer?url=%file_url_encoded%&amp;embedded=true" height="780" width="600"></iframe>