PDA - Sum of Two Characters = Sum of Two Other Characters
You can probably build a PDA for the language $\{p^nq^n\mid n \ge 0\}$. If so, you're almost there. Just treat the a's and b's as if they were all p's and the c's and d's as if they were q's, like this (with details elided): 


Count the a's and b's by pushing a marker on the stack for each one (making sure that all the a's precede any b).
Match them up with the c's and d's: for each one seen, pop a marker off the stack (again, making sure that the input is correctly formed).
If the stack is empty after having read all the input, accept.