1. Find the PDF file in Google Drive

2. Preview the PDF file in Google Drive

3. Pop-out the Google Drive preview

4. Use the More actions menu and choose Embed item

5. Copy code provided

6. Edit Google Sites page where you want to embed

7. Open the HTML Editor

8. Paste the HTML embed code provided by the Google Drive preview

9. Use the Update button and Save the page

https://www.steegle.com/websites/google-sites-howtos/embed-drive-pdf