Partial Solution on How to Regenerate an Invoice (checked on PS 8.1.2):
Run this link in your browser and replace highlighted text as mentioned below: https://your.site/admin/index.php/sell/orders/1234/generate-invoice-pdf
Note:
1. change the name of your.site
2. replace admin with actual admin folder name from your back office
3. replace 1234 with actual order number found in your back office
N.B. this will generate invoice with IN000000 number, but will not update/save it in database
May be somebody knows how to update database with invoice, so that customer can download the invoice from their account page?