The vast majority of portable AutoCAD files on torrent sites or forums are bundled with trojans, keyloggers, or cryptominers. Google Drive’s virus scanning is basic. Downloading a pre-made "AutoCAD 2015 Portable" from an unknown source and syncing it to your Drive is a security nightmare.
The search phrase "AutoCAD 2015 Portable Google Drive" typically refers to users looking for a modified, unofficial version of Autodesk AutoCAD 2015 that can run without installation (portable) and is hosted on or downloadable via Google Drive links. --- Autocad 2015 Portable Google Drive
Even if you find a clean version, expect missing features: 3D rendering engines, plot styles, network licensing, and add-on installers (like AutoCAD Architecture or Mechanical) are usually stripped to reduce file size. The vast majority of portable AutoCAD files on