When you preview a file using our Salesforce to Azure app, it is considered a Read operation. Azure charges for Read operations in batches of 10,000, with the cost varying based on the storage tier (Hot, Cool, or Archive).
Downloading a file, on the other hand, is considered an Egress (data transfer) operation. Egress charges are applied per GB of data transferred out of the Azure region. The cost depends on the amount of data and the destination.
However, if the data is accessed from a different Azure region than where the storage account is located, previewing will also incur Egress charges in addition to the Read operation charges.
For more detailed information, please refer to Azure's pricing documentation.