http://www.steegle.com/websites/google-sites-howtos
From the site:
Instructions
- Find the PDF file in Google Drive
- Preview the PDF file in Google Drive
- Pop-out the Google Drive preview
- Use the More actions menu and choose Embed item
- Copy code provided
- Edit Google Sites page where you want to embed
- Open the HTML Editor
- Paste the HTML embed code provided by the Google Drive preview
- Use the Update button and Save the page
What is crucial is clicking the "Pop Out" icon and then going to "More Actions" to get the embed code!