Thanks for you suggestion.
The way I solve this is kind of awkward. First, I found the plugin works well for root user. Then, I deleted .opera folder in my own home directory and make sure only following two plugin directories are used -- /usr/lib/opera/plugins and /usr/lib/mozilla/plugins. Actually I put nppdf.so in both of them when I was trying to solve this problem by various means, so I have two acrobat reader plugins recognized at the same time. After opera regenerate .opera for me, the plugin works. I still don't know what exactly happened.
When I had such a problem, the pdf files can only be dealt with the pdf plugin by moziplugger. This cannot be changed even if I had nppdf.so in the plugin path. After deleting .opera and restarting opera, I had the freedom to choose between moziplugger and nppdf.so
Next time, maybe I should try using dynamic link.