Maybe you can have Google Drive, Box, Dropbox integrations. You could then request permissions via oauth and upload / retrieve document on the user's behalf. Th

Maybe you can have Google Drive, Box, Dropbox integrations. You could then request permissions via oauth and upload / retrieve document on the user's behalf. This also allows users to upload/read documents using said applications application and yours. User visits your site and wants to upload a document. They see the logos for Box, Dropbox, Drive, and click on whichever they prefer/have an account for. You authenticate then upload the document to a specific folder in a specific namespace in that drive (user_drive_box_dropbox/acme_saas/foo.pdf). For people who don't have an account in any, you can use the same integration to upload documents to your drive's acme_saas_drive/user_id/foo.pdf. This way users have control over their data, and you fan out adding integrations one after the other.