If you are looking for Archicad 15 for a specific legacy project or for learning purposes, here are the safest routes:
But why does a version released over a decade ago still garner interest, and what should you know before trying to run Archicad 15 Portable today? What Was New in Archicad 15? Archicad 15 Portable
If you need to use Archicad, it is recommended to use official channels provided by Graphisoft : If you are looking for Archicad 15 for