Is a dwg file 2D or 3D CAD?
that doesn’t matter if it is a 2d or a 3d file, Autocad handles them the same way. It might be a compatibility problem between your version and the receiver’s autocad version. I often had the issue that I saved the file in the newest default dwg format and the receiver had for example autocad 2000 only and they couldn’t open it. Try saving the file in an older format. Edit: hold on I just see thay the file will be opened only in autocad. What program you are using to generate the file? Is it another version of autocad or something else? If something else then the dwg file may not be compatible with autocad.
