How can I export a selected part (STP filé) from 3D Viewer to STL in order to print ir?

That's not how the 3D viewer in the model Library works.
Download the file, import it to Blender, remove the unwanted portions, then print the desired section.
The 3D viewer in the Library is a VIEWER. It does no editing to the model. You can download from the Library, but you can't edit in the Library.

Dear FredSWUG,

Thanks for your reply!
The thing is... the file.STP format is not inportable to Blender !
(https://grabcad.com/library/videoray-pro-4-1/files?folder_id=534929)
I need somehow to export it to one of the formats accepted by it...

I have already tried FreeCAD but it does not export only the parts I want, but all the model !

Is there a software accepting STP format?
(AutoCAD seems not to accept)

Thanks !
Paulo.

Dear Adam Snow,

Thanks for your reply.
I have found ABViewer14 and managed to export the STP file to OBJ which Blender managed to read.
I am now working on it... hope the OBJ format works well for Blender!

Thanks again!
Paulo.

Export it as VRML (.wrl) blender can open that format.