Each generated invoice will be saved right inside “ywpi-pdf-invoice” folder, which is included in “uploads” folder of your WordPress installation. A new folder with all generated invoices will be created for each calendar year.
Numeration is sequential and begins from the value specified in “Next invoice number” field in plugin settings dashboard.
If you change this value and save new settings, the first new generated invoice will no longer follow the previous numeration and will begin from the new specified value.
You can add a prefix and/or a suffix to the invoice number by changing the invoice name format in “Invoice number format”field.