Noticed this feature recently where a paper document with full of numbers can be stored/distributed over the web as well. May be we can do similar thing for purchase order/Invoice and plain Point of Sale receipts.
Invoice sounds more interesting... We need more work flow (deeper integration into existing documents that consume the Invoices) and support dual mode (Paper containing a way to enable pulling the info online as well - in machine readable data format and human readable format for legal purposes).
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Recently noticed the following software