Is the Post Correspondence Problem with more than two rows harder than the standard two-row variant?
$\text{n-PCP}$ can be reduced to $\text{2-PCP}$ as follows.
Let $M$ be a nondeterministic Turing machine that accepts an $\text{n-PCP}$ instance as input, guesses a solution for that instance, and then checks that it is correct. Clearly, $M$ accepts if and only if there is a solution to the input instance.
Now, let $p$ be an instance of $\text{n-PCP}$. The reduction encodes $M(p)$ as a $\text{2-PCP}$ instance in the standard fashion.