Problem opening a PDF file to work on in Illustrator 2023

Hi,

I was asked to work on a PDF file - to make some alterations.

However, upon opening the file a window popped up with this message:

An unknown shading type was encountered.
An unknown imaging construct was encountered.
To preserve appearance, some text has been outlined.

To be sure - when I clicked ok and the file opened, a good part of the text w as covered with pink boxes (I guess these are the outlines) and they didn't get removed even when I saved the file as an "ai" file.

Any clues?

Thanks,

Vita

P.S. I added screenshots of the original file and how Illustrator displayed it following

the error message: