If you need to upload a PDF invoice, you can do it directly from the Task page, under the Invoices section in the new simplified task view, which creates an 'Additional invoice'. By clicking on the +button, an invoice form will be prompted and you will be able to add all your invoice details as well as upload a PDF if needed for that specific task.
If you want to upload an invoice without a PDF, it can be done through the Invoices tab.
Any questions? Please reach out to firstname.lastname@example.org