ipj to stl conversion is almost certainly searched by users that want to export Autodesk Inventor projects (.ipj) files to STL, but do not have access to the software, which makes them look for alternatives.
Unfortunately, it doesn't seem that any other software can work with Inventor's .ipj files, which means you have to find someone who owns that software and would be willing to do the export for you. Online user communities are usually quiet helpful, so maybe that is your best option.
Updated: September 14, 2023