There are a number of ways that you can add file attachments to a Google Site. For instance, you can add files to the bottom of a page in the attachments area. You can also add a File Cabinet page. However, my favorite, by far, is the Folder gadget.
The folder gadget lets you display the contents of a Google Drive Folder on a Google Site. You add your folder by editing a page and going to Insert > Drive > Folder. Then you select the folder from your Google Drive that you want to add to your Google Site.
For me, the Folder gadget has a number of advantages over page attachments or a file cabinet page. Here are a few of the important differences:
1. Updates: The problem with attachments and file cabinet pages is that every time you update a document, you need to remove the one you had from a Google Site and upload the latest version. If you use Google Docs, Spreadsheets or Presentations, you can log in to your Drive account, and edit the document. Because the file is in a folder that is embedded on your Google Site, the latest version of the file is automatically pushed to your website. This is great for teachers who update a syllabus or other class documents.