Wavefront OBJ is the closest thing 3D has to plain text: a list of vertices, one per line, then faces referencing them by index. It has been readable in every 3D tool since 1990, which is why it survives as the format people fall back on when nothing else will open.
Every conversion below runs in this page, on your own machine. Nothing is uploaded, there is no queue, and there is no account to make. Pick a target format, or just drop an OBJ file above and choose afterwards.