Since the Plugins page has been removed in upcoming Chrome 57, you may find it difficult to manage PPAPI-based Flash and built-in PDF Viewer plugins, if you don’t know, they’ve been moved to content settings. If you want to disable/enable the Chrome PDF Viewer from version 57 onwards, you need to follow the below procedure.
Disabling or enabling Chrome PDF Viewer in Chrome 57+
1. Click on Chrome menu, select Settings
2. Scroll down and click on Advanced
3. Under Privacy & Security, click on Content Settings,
4. Click on PDF documents and check
‘Open PDFs using a different application’. ‘ ‘Open PDF files in the default viewer application’
Done! Chrome Internal PDF Viewer has been disabled.
Note: Chrome blocks Flash by default. You can allows sites to run Flash or manually add sites that can run Flash by visiting Flash section in Content Settings.
Update: Added screenshot of non MD-settings page of Chrome version 57.