Ask HN: Best service to upload user documents securely?

Hi everyone, as part of a side project SAAS I'm building, I want my user to be able to upload documents (PDF files of up to 4 pages I would imagine). Those document can contain sensitive and personal information, so I'd rather not have them on my server for security reasons. Ideally, I'd like an API from a third party service which allows me to integrate a document upload and retrieval from the interface of my webapp, without those documents going to my server.

I must not be searching for the correct thing as I fail to find even one service doing that.

Any advice?