Use the rasp side of the file to shred the PVC surface. Pull the file in many directions to give it an organic look. If the file's teeth get clogged, use a wire brush to quickly clear them.
CAUTION: This step will make sharp ridges on the surface of the PVC. Try not to slide your knuckles along the pipe until you have smoothed it out.
Once you're happy with the texture, use the double cut (smoother side) of the file to remove the shavings and sharp ridges on the PVC. Give the plastic a few extra rubs with the sandpaper.