The problem appears to be in the line
oProcess.StartInfo.Arguments = InputFileNames + "output" + OutputFileName
There are no spaces in the argumentsAFAIK the correct syntax for pdftk would be
pdftk C:\Users\sathishr\Desktop\Small\a_20160310_*.pdf cat output C:\Users\sathishr\Desktop\Small\Test_US.pdf
so I think that line should be (note untested)
oProcess.StartInfo.Arguments = InputFileNames + " cat output " + OutputFileName