Why is Adobe Acrobat using only one core for a heavy task, and how can I make it perform faster?
This is an Adobe issue. Namely, Adobe Acrobat Pro is not multithreaded and therefore cannot take advantage of multi-core CPUs.

Unfortunately, Adobe has been rather slow in implementing this across most of its software. For example, I refer you to Why is Illustrator throttled to 2% speed when it's in the background? in which Allan provides a great rundown on this (although in relation to Illustrator).

In terms of Adobe Acrobat, I refer you to Acrobat and multi core cpu support. Although this is nearly a couple of years old, it is still indicative of the current state of affairs.

In summary, there really isn't anything you can do except submit feedback to Adobe about this and hope they implement this soon.