I am unable to search many of my .pdf files on my ipad pro. Is this a known issue?

I am a student and I am using acrobat to read my textbook pdf files on my ipad pro.  I annotate and highlight them.  I also need to be able to search them. I am able to search some of my text books and files, but not all of them. Why is this?  The files that I cannot search on my ipad, I AM able to search on my PC.

Thanks for any help.