How can I configure Comodo to open a PDF in my Window’s PDF viewer rather than its internal one?
Thanks
Hi and welcome. 
You can go to chrome://plugins and disable Chromium PDF Viewer.
I guess you are aware that Chromium PDF Viewer is more secure than most external ones.
Secure in what sense?
In the sense that it is very hard for a malicious PDF to do anything malicious. Chromium PDF Viewer’s process is very restricted by Chromium’s sandbox, just like the renderers’ processes are.
On Windows 8+, you can make it even more secure than it is by default, by going to chrome://flags/#enable-ppapi-win32k-lockdown and enable Win32k lockdown, and you can also enable AppContainer: chrome://flags/#enable-appcontainer
In the attached image you can see the PDF Viewer running (the blue, selected process) with AppContainer enabled.
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