[giaour]

R2 would probably be significantly cheaper because they don't bill for bandwidth, but S3, GCP storage, and Azure storage would likely be in the same ballpark. Blob storage is a commodity service, so the major providers mostly match each other's prices and features.

If you are already using a particular cloud provider for the web application that will be accessing these documents, you should use their blob storage unless you have a compelling reason not to. Communication between a cloud provider's compute and storage services within the same region will have minimal latency.