Unknown Imaging Construct Work-Around

I'm using Adobe Illustrator 26.0.3 on a 2013 Mac Pro using Monterey v. 12.2.1. I work with a lot of client provided PDFs which I'll typically need to bring into illustrator to convert certain colors to Pantones for printing purposes. Sometimes when opening one of these PDFs I'll come across the error "an unknown imaging construct was encountered" and opening Illustrator to find most or all of the text blocked out and uneditable, similar to a rastered image. Is there a work-around that would allow me to edit the colors as needed?