I have never found a way to do full text searches on IOS. Of course, it could just be a case of “I’m doing it wrong”.
When searching for a replacement for Evernote, I uploaded several types of files (txt,pdf,rtf, etc.) to both iCloud & Google Drive. Almost immediately after syncing I was able to find files on Google drive (using their app) by searching for keywords that appeared within the document, but not in the filename. I was never able to locate them on iCloud by searching for the contents of a file (in Files or IOS Spotlight), even days after they were uploaded.
Recently I was searching for a document containing the name of a friend, the Google Drive app prompted me with a “Did you mean:” suggestion when I mistyped his last name. I’ve been very pleased with G Drive.