A group of Microsoft Products and technologies used for sharing and managing content, knowledge, and applications.
Hi Tong2013,
To insert a SharePoint document as an iframe, we recommend you get the embeddable link following the methods below: 1. Open the document in the Office online > File > Share > Embed.
Enable the guest link on your site > create the embeddable link based on the guest link.
Also you may refer to this article to embed your Excel workbook in external sites.
Meanwhile, you may take in consideration that this might be related to security. You may contact your admin because you have to first allow anonymous access to the entire site collection in the collection settings
Regards,
Moja