You can enter a Comment regarding the invoice under the Comment button.
If you have installed the supplementary function Document Viewing, you can use the Document Viewer button, to the right of the Comment button, to link external documents to the invoice. If for example the invoice was attached as a PDF file to an e-mail, you can drag the PDF file from the e-mail message and drop it on this button in order to link the file automatically. You can then view the invoice using this button.