Try to link them when placing and not embed them.
Thanks for your reply.
How can I link? How can I do that exactly?
Anyway, I saved the .pdfs as .png files and imported those to Illustrator and not the .pdf.
There's an option in the Place dialog to link files. Just put a checkmark there.
When placing a PNG there shouldn't be a warning of missing fonts.
But PDF quality might be better, since it might contain vector artwork and real text.
Thank you very much Monika! Yes, the keyword was 'Place'.