You can embed Google Documents, Spreadsheets, Presentations, and Forms into your Google Site in three simple steps:
Select "Insert" on the top of the menu located on the right side of your screen.
Scroll down and select "Docs," "Slides," "Sheets," "Forms," or "Charts".
Select the item you are looking for and resize as needed.
Note: The inserted object will be view-only to the public. However, when the contents of the objects are updated at its source, it is updated automatically within your site.
Another Note: Anyone with editing access (you can choose to grant this, or not, when you create the file) to your embedded file will be able to view a link under the embedded object that will allow them to open an editable version of the object.