How can I join / add pages to an existing entry?

There is no specific feature available yet for adding multiple PDF to one entry, but you can make use of a trick to join addidional pages to your existing entry. Just open the PDF and drag additional pages to the thumbnail view of the Preview app. Then save and refresh the Receipts view by selecting another entry and then select the previous one again. This little video (on a German desktop) illustrates the steps: