That's because you are printing from the screen.

What you should do is to click on the Create PDF button in the bottom-right corner.  Make sure you allow popup from the site.  A new tab will open with the PDF you can save or download.

Note that this is purely a glorified wordprocesor.  Nothing get saved anywhere.  This means you can't retrieve a record of your "invoice" or do any compilation.

A better option would be to use QBOA by selecting Invoice via QuickBooks instead of Invoice.  Otherwise, you can always use your own accounting software or your favorite payment processor.