I can´t open the OBJ files

I have created my first obj but neither copycady nor powershape, they are able to open it.
What could be the problem?

The problem would be that you did not meshed your scan , or you trying to open point cloud .

The name " mesh" should appear in the obj’s name after export.

When you export meshed OBJ , you always get 2 files , one is a mesh and second is a fuse ( point cloud)

I have limited computer knowledge but was wondering what program is best for opening my saved scans on my PC. I have saved the Mesh files in all 3 formats to be safe obj, stl, ply. What do I need to view them and import to a cad program or a 3d printer ?

Your 3d printer slicer should be able to deal with obj or stl files no problem. Prusaslicer and Cura both can.
Any cad package will be able to deal with stls.