Hi @ideefabriek,
The document number and date is stored in the order data, and this info is not deleted when you remove the plugin for the site. You can find the invoice data under these key within the order data:
_wcpdf_invoice_number: The invoice number_wcpdf_invoice_date_formatted: The invoice date (formatted)
However, the PDF document itself is generated on the fly, but not stored in your site by default. Only the documents attached to the WooCommerce notifications, are temporarily stored, but they are deleted 7 days later, by default.
That said, if you really need the PDF documents, and have the Professional extension, you can use the Bulk export tool to export all your PDF documents in a zip file.
If on the other hand, exporting the orders and invoices info in a CSV or XML file is enough, there are several export plugins available that integrate with ours. For example, you could try a pretty good free plugin called Advanced Order Export For WooCommerce from AlgolPlus. Another option (paid) that also collects the fields of our invoices and is a little more intuitive is: Store Exporter Deluxe for WooCommerce by Visser Labs.
Below you’ll find a short video explaining how to export both the invoice date and number, using the first plugin I recommend above (the free one):