Solution :
I have checked application web.config file and found that <authorizedType Assembly="DocToPDFActivity, Version=1.0.0.0, Culture=neutral, PublicKeyToken=2478de059b6051f4" Namespace="DocToPDFActivity" TypeName="*" Authorized="True" /> is configure in it.
So Just i have added it and reset IIS.
After I have checked Workflow action UI and it appears.
Cheers.