Try to just drag-and-drop them! Should work just as well.
I have no problems with Illustrator PDFs, but I'd advise to turn off Illustrator's Save illustrator editing options (or whatever it's called) when you save to PDF. I guess that's what's confusing Archicad.
If you're not able to save compatible PDfs from Illustrator, a workaround might be to Print (using the AdobePDFprinter utility) to a PDF file.
AC4.1-AC26SWE; MacOS13.5.1; MP5,1+MBP16,1