When the figure's property 'RendererMode' is set to 'auto', many 3D objects enable the OpenGL renderer. This renderer cannot create a vector output, such that you get an pixel image. Solution: Set the renderer manually to 'Painters':
figure('Renderer', 'Painters'); % At the creation
Or before the export:
set(gcf, 'Renderer', 'Painters');
Note: This problem has been discussed repeatedly in this forum, such that searching for "pdf pixel" would be faster than waiting for someone, who answers this again.
