Thank to all of your suggestion.
However, I am still not able to remove Workspace Submit button out. Currently, I have 2 indirect submit buttons (Approve, and Reject button) on PDF Forms and 2 Workspace Submit buttons appear on screen.
I added one process submit fields in PDF Form and also hide it. Two indirect submit buttons had been added with the script for interacting with hidden process submit button. I had modified Render PDF service by removing Workspace service. Both buttons in PDF and button in Workspace were rendered.
I do not know what I made mistake. 
Again, thank you.
Mac