Hi,
In this case i would associate document to customer(account) entity which will be customer_document , when customer will upload document i will set this uploaded document to that association.
I would then create 1 page for customer ,in which there will be listview for all customers and you can give download button per customer and set its visibility condition as customer_document should not empty.
in that download button call a microflow with account entity as parameter , by this parameter retrieve its document and put a download action and download it.