For proving language $\{a^nb^mc^nd^m \mid n,m > 0\}$ is not context free. Do I have to use $z = a^pb^pc^pd^p$ as pumping lemma string where $p$ is pumping length?
Or do I have to use a string that would reflect than $n$ and $m$ does not have to be the same length?
Pumping lemma lets you construct new strings belonging to the context-free language.
To prove that the language is not context-free you can take some string $s$ from L, use pumping lemma to construct a new string $s1$ which must belong to $L$ (because you used pumping lemma to construct it!). Then you show that $s1$ does not belong to L => contradiction.
Choice of original $s$ is yours. So yes, you can use $z = a^pb^pc^pd^p$ in your proof.
