Once you save the invoice form, you can access it at any time (via Accounting > Accounts Payable > AP Transactions or via 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 are using our Documents to Process feature, you cannot 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.