I'm using PCL and I'm able to visualize the direction of the normals in real-time using PCL Visualizer.
When I store the point cloud I'm getting stored only the points but not the direction of the normals.
How could I store the direction of the normals in a PCD file, in order to visualize them using pcl_viewer?
You have to store the point cloud with the right point type.
I'm guessing that you are sotring the cloud as pcl::PointXYZ, but what you should to is to store the cloud with a point type that support normal information, i.e pcl::PointNormal; or just store the normal information alone: pcl::Normal.
Check this link for information about what Point Types are supported on PCL: http://pointclouds.org/documentation/tutorials/adding_custom_ptype.php#id3
And chek the section 'Normals and other information' of this link for information on how to visualize the normals: http://pointclouds.org/documentation/tutorials/pcl_visualizer.php