Thanks Sanford. I've checked and:
'Download PDF files instead of automatically opening them in Chrome' is Off
The PDF loads if I manually put it into a new tab
PDF doesn't load if I refresh the page the JS opened
Although the issue only seems to be occurring in CodePen on Chrome - running Version 73.0.3683.75 (Official Build) (64-bit). It does work if I add it to a Marketo LP, which I guess is good enough for me. But strange it's not working in CodePen.
See video of CodePen demo on Chrome 73 x64/Windows 10: CrossBrowserTesting Video Recording
Thanks for the tips, Sanford. But still doesn't seem to be working, but I'm guessing it must be something local to my machine: Video recording
How could it be local to your machine if it's happening on CodePen using the CBT lab?
Test the "debug" mode on CodePen, not "full": https://codepen.io/jwright_tmf/debug/oVPVWE
It was just an IFRAME-in-IFRAME security thing, then. Debug doesn't use an extra frame.