In case this is helpful to someone else, I saw the "Couldn't load plugin" error for pdf on only one web site (other sites seem to work fine). The site is http://geoscienceworld.org. You might need a subscription to try it for yourself - not sure. Here's a more specific example of a pdf I could not see with the plug-in: http://ammin.geoscienceworld.org/content/101/7/1704.full.pdf+html
To resolve the problem, I when into 'Settings > Plug-ins > Manage individual plug-ins...' and disabled Chrome pdf Viewer (disabled both plugins listed under "Chrome PDF Viewer"). Now it appears that my own Adobe plugin is doing the job instead, and I can load pdfs without problems. I have both Acrobat Pro 9 and Adobe Reader DC (free) installed on my system.
This is my Opera version info:
Version: 39.0.2256.48 - Opera is up to date
Update stream: Stable
System: Windows 10 64-bit (WoW64)