Summarize the following:
" This error occurs when you try opening a newer DWG file using an older version of AutoCAD. For example, if trying to open a DWG file in AutoCAD 2012 that was created in AutoCAD 2015, try opening the file using AutoCAD 2015. Third-party apps integrated with AutoCAD may interfere with opening DWG files. A DWG file may be corrupt if it originated from sources outside of AutoCAD or Autodesk products.
Try opening the DWG file using a newer version of AutoCAD if you receive the error "Drawing file is not valid. Quit any third-party applications running inside AutoCAD if you cannot open a DWG file. Verify that the DWG file originated from AutoCAD if the file fails to open.