The invoices that you print from your account are historical invoices. They show your payments and, if applicable, the taxes we charged you on those payments in a given period. Because each invoice includes our tax ID as well as yours, is legally valid and will be accepted by your local tax authorities.
If you're concerned that these invoices might not be legally valid without a stamp or original signature, please know that our online invoices comply with the European Union requirements given in the current VAT directive, Article 226, Council Directive 2006/112/EC. The directive stipulates the formal requirements for a valid VAT invoice, and it's been incorporated into the domestic VAT legislation of each EU member state. Please consult your local legislature or tax advisor if you have questions.