PDF's not loading in browser for some users


We set UBE Content Disposition to false with great success. All but 3 of our users now have PDFs load directly in a browser tab as desired.
However for those 3 users JDE wont load the PDF in the browser. it only shows the browsers Open/Save dialog.

Things we have tried:
Cleared browser Cache
Different PC's
Both IE 8/9/10/11 and Chrome 44.0.2403.125 m (64 Bit)
Different Windows Users.
both Foxit Reader and Adobe Reader

The three users in question use different roles so i don't believe its a role issue.
i believe its a user setting in JDE, but after a lot of google searching i have come up short.
any ideas?
Have you checked if the "Launch programs and files in IFRAME" option is enabled in IE Internet options? Thats under the custom security settings.
The three users - are you having an issue with those JDE users on a workstation that works for another user ? Ie, is the issue to do with the workstation or the E1 user ID ?

Its more likely to do with the workstation, and therefore the default file settings.