The portal users can easily view the Sharepoint documents in the portal if the admin has enabled the SharePoint. After enabling SharePoint here is how your customers can view the SharePoint documents.
Frontend
Your users need to log in to the portal. After logging in they have to navigate to the More Documents, and they will land on the Documents page. Here they will get a list of documents.
The portal users can also upload and access any files with valid types in SharePoint. The Portal users can upload any type of file like Audio, Video, Photos, Zip files, etc.
Note: The portal users can upload the maximum file size of 5 MB.