Hi - thanks for testing.
I can download the PDF from the link you have provided no problem.
But the problem persists inside the Xero.com application, with the same error.
I’ve updated to:
Version 0.60.45 Chromium: 72.0.3626.109 (Official Build) (64-bit)
Message: Unexpected server response (500) while retrieving PDF
When I tried from a banking site (anz.com) the error code was “Invalid PDF structure” instead.
I’m assuming it as something to do with the credentials behind a secure site.