Google Docs offers an undocumented feature that lets you embed PDF files and PowerPoint presentations in a web page. The files don't have to be uploaded to Google Docs, but they need to be available online.
Here's the code I used to embed the PDF file:
<iframe src="http://docs.google.com/gview?url=http://infolab.stanford.edu/pub/papers/google.pdf&embedded=true" style="width:600px; height:500px;" frameborder="0"></iframe>
but you should replace the bold URL with your own address. As I mentioned, the document viewer works for PDF and PPT files.
Some other sites that offer similar features: Zoho Viewer, PdfMeNot.
Update (Sept. 25): The viewer is now available at https://docs.google.com/viewer. You can use it to embed PDF documents, PowerPoint presentations, and TIFF files.