PDF incompatible with version of photoshop help

Hi,

So two files I can no longer open in photoshop. They are large designs with 100+ layers 500mb+. i always save as a pdf this is the first time ive had an issue.

i beleive the files where made in either version 24.7 or 25. they were made a few days ago. now i cant open the file without the files is " incompatible with version of photoshop" message. I tried openeing on my old version 20 too and i got the same message. ive tried on 2 different devices. windows 10 pc and my windows 11 laptop. the files are saved to an external samsung ssd. ive tried opening in ilustrator , it opened but all the layers are merged apart from some text layers but im not familar with ilus. the pdf can be viewed as an image in the expolorer.

the full message is "could not complete request because the file is not compatible with this version of photoshop". 

can anyone help please?