Hi Nelson and thank you for your response.
iOS behaviour is fine.
Android does not behave as you describe on any of my test devices. On four different test devices using Android, the link opens fine in InAppBrowser, and the user is free to navigate, but nothing happens when the user clicks on a link to a pdf file.
I've recorded the behaviour for Android using each of the possible 'Target' parameters for InAppBrowser in the comments sections of the diagram above.