It doesn't matter whether the .ai file is 1.5GB or 5MB, whether it's complex or simple; I always get the "The file cannot be opened." critical error.
I use CorelDRAW X5 not Adobe Illustrator but there should not be any issues since CorelDRAW opens and saves to and from AI without any issues whatsoever. Except for .ai files exported from ArcMap. There seem to be no issues with the PDF export. It opens in CorelDRAW fine and I can manipulate and edit vector graphics. However I would like to be able to use the AI output for my publishing work. (Layers, named objects, etc.)