Batch convert ai files to pdf / png etc

You can use GhostScript for batch output .ai to various formats, i.e. transparent PNG:

gs -dNOPAUSE -dBATCH -sDEVICE=pngalpha -r300 -sOutputFile=out.png in.ai

Where -r300 is dpi resolution.

For available devices with which you can convert .ai format, reference documentation.

A typical .ai file (from version 9 up), saved the normal way (with PDF compatibility) does not need any special conversion (in most cases). For example, if you take a typical .ai file ...as I said, saved with PDF compatibility which is the default ... you can simply add .pdf to the end, and then open it in your default PDF viewer. No special conversion needed. On the other hand, if the file is not saved with PDF compatibility, the only option you have is to use Illustrator. All ai to pdf converters I have found require that the ai file have the PDF data to "convert" ... which they aren't really doing since its already a PDF to begin with (just change the extension to .pdf and see).