I am currently working on Autonomous Ground Vehicle using Pixhawk 2.4.8. I am planning to work on obstacle avoidance using Lidar for it. So, can I use this Pixhawk or do I need to use a separate Arduino UNO for obstacle avoidance?
Just use the pixhawk
Maybe you will have to make a custom build to run the obstacle avoidance algorithms with this board, it has some memory limitations so maybe it will not be available on the default build.
While there are a variety of hardware configurations for these generic Flight Controllers what has been common is the processer has 2mb flash. So if fmuv3 or Pixhawk1 firmware is used there should be no features limited.