If you have tried to load a PDF into your Google Site - you have found out that, well... you can't.
However, there is a solution.
1. Upload your PDF to your Google Dive.
2. Make sure to change your sharing settings.
3. Copy the url.
4. Return to your Google Site and the area you wish to have the PDF.
5. Select embed URL and paste in your URL you copied before.
6. Boom - there is your PDF.
Embedded below is a GIF followed by a 2 minute YouTube tutorial.