Any recent Illustrator file is a PDF file - even though it may not look like one. You could rename an .ai file to .pdf and then open it in Acrobat. However, I would not recommend that. The better way to get from. AI to PDF is - as David suggested - to export from AI to PDF.
If you can tell us more about what you are trying to accomplish, we might be able to provide more information about how to best do this.