If you ever use Chrome browser to view PDF document, you probably aware that Chrome has it’s own PDF viewer plugin. This allowed user to view any PDF document using iDF document, you can easily disable to plugin in chrome://plugins by un-check the plugin checkbox in that page.
But this has feature has changed drastically since Chrome upgraded to version 56 and above. With the latest Chrome browser, you can’t disable the PDF plugin anymore in chrome://plugins. As stated in chrome bug tracking, they mentioned that chrome://plugins will be depreciated soon.
And now, Chrome has moved the setting to Content Settins page instead.
Settings > Show Advanced Settings > Content Settings. Tick the “Open PDF files in the default PDF viewer application”, then you would be able to view PDF document using your own default application.