What .ai files are
AI is the native file format of Adobe Illustrator, used for logos, packaging, posters, labels and illustrations. Without Illustrator, most people — clients, printers, colleagues — cannot open an .ai file, which makes reviewing and sharing designs difficult.
What happens when you convert
When Illustrator saves a file with “Create PDF Compatible File” turned on (the default), it stores a complete PDF version of the artwork inside the .ai file. This tool copies that PDF out unchanged. Because nothing is re-drawn, the vector shapes, text and colours stay exactly as in the design and remain sharp at any zoom level.
When conversion is not possible
If the designer turned off PDF compatibility to save space, the .ai file contains only Illustrator's private data and no converter outside Illustrator can read it reliably. In that case you see a clear message explaining how to save the file again.
Common problems and how to fix them
“Saved without PDF compatibility”: ask the designer to open the file and choose Save As → Illustrator with “Create PDF Compatible File” ticked, or to export a PDF directly. Fonts look different: the fonts were not embedded — the designer should outline the text or embed fonts. Several artboards: they all appear as pages in the PDF if they were included in the compatible PDF.
Good habits
Share the PDF for review and keep the .ai file for editing, ask for print-ready PDFs from designers for printing, and check colours on a proof before large print runs.