Due to phasing out of support for NPAPI plugins, even though you’ve disabled Chrome native PDF viewer in Google Chrome, it still doesn’t allows to open PDF files inline through plugin of Adobe Reader installed on your computer. Chrome blocks Adobe PDF plugin by default and displays "Adobe Reader is not allowed" error and asks for your permission to run it. And hovering over icon in address bar shows "the plugins were blocked on this page "
For that time, you can select always allow plugins on that site or right click on Plugin icon and select "Run this plug-in". Both actions don’t solve the problem permanently and when you get another PDF file in Chrome you’ll encounter same error.
At present, you can find this happening in Chrome Canary and soon on beta and stable channels from January 2014.
Fixing Adobe Reader is not allowed or chrome blocks third- party PDF plugins, to allow them always
If you don’t like viewing PDF files in Chrome PDF viewer and wants them displayed from third-party PDF Reader, then you need to follow below steps.
1. Visit chrome://plugins in Chrome.
2. For Adobe Reader Plugin, select "Always Allowed"
Similarly, if you’ve installed Sumatra PDF or Nitro PDF Reader for Chrome instead of Adobe Reader, for Chrome to always open PDF files in them you need to visit chrome://plugins page and need to select "always allowed".