Word Documents, PowerPoint presentations or Excel spreadsheets - these types of documents are usually not generic enough to be of general interest and aimed towards mass distribution.
Like the YouTube video player, iPaper is a document player that lets you embed PDF, Microsoft Office documents, spreadsheets and presentation in web pages. You also get to monetize documents via Google AdSense.
While Scribd and Slideshare also support PDF files, Issuu allows you to upload large PDF files (upto 100 MB) and their inline PDF player looks absolutely amazing. You even get to decide the colors and the PDF player dimensions.