Hi Johan,
Firstly - I'm not 100% sure you can go from a popup to another popup via URL on save. That's something I'll look into.
URL on Save is, as the name suggest, a URL.
Secondly you're calling 'MailboxesReportWithOptions' as the URL on Save, but that's a Report procedure. So that will never be a popup. Assuming the popup-to-popup worked, this Save as URL would need to be a Form - and that form could contain the PDF (using the media control demonstrated elsewhere.)
So maybe re-work the example so you are doing a URL on save to a Form, which contains the report - and let's go from there.
Cheers
Bruce