I needed to convert a topic of rosbag2 file to a folder of pcds. I’ve searched but somehow missed this great ros2bag_tools tool. And created this small single purpose package 
If you need to convert a ros2 bag file to pcds, you can use this package, it’s pretty basic.
Assumes the point cloud in the bag has at least the regular float x,y,z,intensity fields.
If I’d found it first, I would’ve probably just forked ros2bag_tools and edited exporter/pcd.py to fit my naming scheme.
Related post: ros2bag_tools: A set of usesful tools for ros2 bags