Cannot search text inside pdf files on the network drive

I am using DO 12.9 x64 and I have about 500 pdf documents on the network drive that I had to process. I tried to search a simple text inside pdf files so I can know which files contains that text, but "Search" in DO does not find any (I am not using any other filter except "Name matching"). I tried also using Microsoft file explorer on the same files with the same search and it found some pdfs.
Please advice what can I do?
Best regards.

If File Explorer worked, try entering the same search string you used there into the search field at the top-right of Opus. Does that work?