Afanche 3D Viewer for phone
This is a great professional level 3D file viewer for iPhone. The apps provide 3D visualization for many popular 3D formats including STL, STEP (.stp/.step), JT, 3MF, SKP (SketchUp), IGES (.igs/.iges), SCDOC (SpaceClaim), X_T / X_B (Parasolid), Solidworks( .sldprt, up to version 2018), PLY, OBJ, 3D