Invoices are saved in the “Invoices” folder by default. You can find this folder in “wp-content > uploads >ywpi-pdf-invoice” of your WordPress installation.
In case you want to set a custom path for saving, you can use [year], [month] and [day] as placeholders (according to the date that is generated).
Path example
If you gave in: “Invoices/[year]/[month]” an invoice generated on “17 February 2016” will be saved in the path “invoices/2016/02”.