Once you save the invoice record, you can access it at any time (at Accounting > Accounts Payable > AP Transactions or at Accounting > Accounts Receivable > AR Transactions). On the record, you can click the attachment to view it:
There, you can right-click the image and use the "Save as" function to download it.
If you're using our Documents to Process feature, you can't download invoices awaiting processing directly. You must use the feature to process the invoice and create a record. Once you do so, you can download the file using the method above.
We are currently working on adding the ability to download unprocessed invoices directly from the Documents to Process feature.