We don't have a lot of internal documentation on Chinese strategic thinking when it came to aiding the Pakistanis (the Chinese are pretty opaque about such matters). There are arguments that can be made based on what it appears to have "gotten" them and why other states have aided in the proliferation of others. Matthew Kroenig, who has written on the latter (in his Exporting the Bomb) argues that one ought to see the real "target" of Chinese assistance to Pakistan was India — with a nuclear Pakistan, India has almost all of its military, diplomatic, and strategic attention focused to the northwest, not to the northeast. In other words, by "distracting"/complicating another major power in the region, China is given more freedom of action for its own interests. You can see, for example, why China (and Russia) are more or less OK with a nuclear North Korea for the same reasons — it focuses the US attention, giving them more latitude to do other things (and in the case of North Korea, gives China more power as the "broker" of diplomacy in the region).