Customer Portal for Storing Documents

My project requires a customer portal to store documents. The portal will allow us as the business to create a user account for our client, send out login details and then upload PDFs and securely store them in a login-protected environment. Users will be granted full access to view, read and write these documents, but must use a secure login to gain access to the portal. Us as the business will have full view, read and write abilities for all clients. There must be a list of all active users for us the business to see. It is essential that the portal be highly secure and reliable, as it will store confidential customer data. The successful bidder for this project must have experience in creating custom portals with secure login access and other features as specified. All bids should include details of experience and skill-level in creating such custom portals. We would prefer to use something like Microsoft PowerApps or Google FileStore, but are open to suggestions and custom solutions.