Sorry for not being clear. I meant , have you tried registering new DLL with MS Windows, in the paths to which you copied them. For example, in command prompt:
regsvr32 "%LR_PATH%\bin\Firefox\plugins\nppdf32.dll"
regsvr32 "%LR_PATH%\bin\gecko\plugins\nppdf32.dll"
Again, this is only a hunch. Please reply with the result and any additional detail you may obtain if this does not correct the issue.