How to convert a file ".Obj" in a ".Stl" ?
I need a free software please !
I don't know any free software. But I can help you in this regard.
You can send me the file so that I can check and see if I can convert it for you.
Try CAD Exchanger, desktop or Cloud (https://cloud.cadexchanger.com). The former has a free 30 days eval version, the latter - a free plan.
Other formats are also supported.
Our Free Viewer (for STL & OBJ) provides the ability to convert from OBJ to STL as well as STL to OBJ: https://www.solveering.com/pages/software/software.aspx. If you need more capability (convert to B-REP, remesh, etc.), give the InStep Studio application (https://www.solveering.com/instep/instep.aspx) a go with a free trial.