I need to batch convert AI files to PDF files, but Illustrator SDK can only develop Illustrator plug-ins.
Is there a SDK that doesn't run Illustrator?
No there is no SDK than runs without Illustrator.
If the files are saved with the "Create PDF Compatible File" option turned on (which is the default setting), you can just change the file extension from .ai to .pdf.
If you just want to save AI files as PDF in an automated way, you can record saving a file to PDF with the Action panel. Then you open all the PDF files and run the action.
Alternately, you may be able to do this with ExtendScript. I suspect you can easily iterate over a directory, loading every AI file and saving them as PDF. The last bit is the only question mark; I've never tried to do that, but I wouldn't be surprised if it had that capability.
Lastly, you could automate it with the Illustrator SDK in a plugin. But bottom line here is there's no way to do anything with an AI file without Illustrator. If that's a deal breaker for you, you're out of luck.