Use simplification filtering by
Filters > Remeshing, Simplification and Reconstruction > Simplification: Quadratic Edge Collapse Detection
Then, MeshLab pops out the following GUI.
Via the GUI shown below, I set
- target number of faces: smaller
- Preserve Boundary of the mesh: checked
- Preserve Normal: checked
- Preserve Topology: checked
- Planar Simplification: checked
