Assign more than 1 CPU/core to a process

Unless specifically designed to do so, software cannot generally use more than one CPU core, and cannot in general have extra added.

(Writing software to run on multiple cores concurrently is significantly more complicated, as you cannot guarantee the order in which tasks will finish, so you have to guard against all manner of problems caused by multiple CPUs trying to access resources concurrently, or events occurring in a different order to that you expected).

That said, PDF cracking is a task that should be parallelisable. It appears you can find a modified version with multi-core support, but it looks like you'll have to compile it from source.