How do I save a
ListLinePlot graphic as an Adobe Illustrator file (with extension
Mathematica cannot export to Illustrator format. However, it can export both to PDF and EPS, which are natively supported by Illustrator. (In fact,
I believe the AI format is based on PostScript too, just like EPS the AI format used to the based on EPS but is now based on PDF.)
Try using PDF or EPS instead.