As I understand, the plugin creates a new folder each year?
No, only the document numbers are reset annually.
Does the plugin delete an invoice when its order deleted? So when I delete an order, the plugin deletes the invoice, right?
It depends. If you trash an order, the document data is retained in the order’s metadata. However, if you delete the order permanently, the document data is also deleted.
Keep in mind that permanently deleting an order with a document number that isn’t the most recent may create gaps in your numbering sequence. If the document has the latest number, you can fix the numbering by decreasing the next number by 1 under PDF Invoices > Documents > Invoice > Next Invoice Number.