To import your invoices Launch the wizard Invoicing -> Vendors -> Import Vendor Bill.
In there you can either directly the QR code of the QR-bill or you can import a PDF/image file.
If it's the first time you import a Bill from a Vendor the wizard will ask you to create the partner and to set an import configuration for that partner. A configuration let you set which expense account you want the invoice to be imported with.
To quickly import a series of invoices, use the button Import Next Invoice from the Vendor bill form.