how to convert .stl file into surface on any software?
Design X (formerly rapidform) has an "auto surface" tool that will help convert a mesh model into a surface.
The end results is usually pretty good, but it depends on what your goal is.
If you need a full model, with smooth surfaces, and features that can be edited, you'll need to make a new file. You can use the mesh data as a reference to speed things up a bit, but it can be a somewhat difficult process.
There is a difference between a point cloud, a mesh model, and a surface model.
You have points now. Once you get them to a mesh, you'll be able to calculate surface area.
Surface models are the next step up in complexity.
If your data is not top secret, upload it here and I'll see if I can make a mesh model.
If it is a little secret, you can send it to me directly, and I'll send it straight back to you.
If it is top secret, you'll need to obtain a program like Geomagics Control, or Design X to convert the point cloud into a mesh... SolidWorks MIGHT be able to do the task with its Scanto3D plugin, but it has never worked well for me.