Link through to Sharepoint/Office365 area for shared documents.

This page can be easily password-protected using WordPress, to provide basic security or a deterrent to the casual viewer: one password that you’d share with your membership.

For greater security, you might use Microsoft’s own security from Sharepoint: with Sharepoint, you can specify (by email address) who can see which folders/documents. See https://sharepointmaven.com/how-to-share-files-and-folders-externally-in-sharepoint/.