Adobe Illustrator Artwork (AI) is a proprietary file format developed by Adobe Systems for representing single-page vector-based drawings in either the EPS or PDF formats. The .ai filename extension is used by Adobe Illustrator. The AI file format was originally a native format called PGF. PDF compatibility is achieved by embedding a complete copy of the PGF data within the saved PDF format file. This format is not related to .pgf using the same name Progressive Graphics Format. The same “dual path” approach as for PGF is used when saving EPS-compatible files in recent versions of Illustrator. Early versions of the AI file format are true EPS files with a restricted, compact syntax, with additional semantics represented by Illustrator-specific DSC comments that conform to DSC’s Open Structuring Conventions. These files are identical to their corresponding Illustrator EPS counterparts, but with the EPS procsets (procedure sets) omitted from the file and instead externally referenced using %%Include directives. Source.