You'll certainly need to create proper invoices if you want to delare the work you have done. The invoices produced by Upwork don't contain all the info required by the Spanish tax authorities. You'll need to get the clients' addresses and tax ID numbers, for example. If any of your clients are Europe-based, you may need to pay VAT and, if a business based in Spain, IRPF.
In other words, you need to treat Upwork clients just like any other client!