What is the most popular file format used for sharing AutoCAD files?

2D  -  DXF, DWG both Autodesk AUTOCAD formats.

PDF, TIFF are very widely used. 2D data is typically detail drawings, but might actually be operable geometry, such as a sheet metal flat pattern.

 

3D  -  IGES and STEP,

..............Of course DWG

Agree with my Mentor. Mr Yaquob.

dwg, however you can convert these files into pdf's a specially when you send them via email

DWG

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dwg files, you can also save your AutoCad files in a plt format

dwg......................................................................................

often design offices use PDF to send file to client but if clients engineer demand for softcopy of drawing then they provide .dwg and use .Jpeg and other pictures formats to share it on social media etc.