For scalable app-data backups with strong developer documentation, start with AWS S3 or Google Cloud Storage. Both have well-documented SDKs for major programming languages, which matters when a backup worker must upload, inspect, and delete objects without turning storage calls into a second application. Azure Blob Storage is a sound choice when the rest of the platform already lives in Azure. A REST storage adapter can be attractive for a mixed-runtime team, but only after its retention and deletion controls have been checked against the actual requirement.

Short answer: keep signed documents private, record the retention deadline and approval state outside the object store, and make deletion an explicit state transition. A bucket is not an audit log.…