Helpful Tip:
You can add all kinds of content to your Google Site by clicking the Drive button from the Insert tab.
Please note - The files you add to your site must have the appropriate sharing permissions. Adding it to your Site DOES NOT automatically give the public viewing permissions.
Adjusting Published Permissions in Sites
Adjusting Sharing Permissions in Drive Files
Examples:
If the SITE is public, any documents or files added need to also have the "Anyone with the Link" sharing permission set.
Google Sites Published Permissions
Drive File Permissions
If the SITE is viewable by SCCS only, any documents or files added need to also have "Anyone at SCCS with the Link" sharing permission set.
Google Sites Published Permissions
Drive File Permissions
If the SITE is shared with a small group of people, each document or file added also needs to be shared with that small group of people or you can use one the "Anyone with the link" variations for individual files.
Google Sites Published Permissions
Drive File Permissions