alexmvp's post in Looking for iPhone app to search text in Excel document was marked as the answer
Ok @simon here ya go. Tested it and it works well. It's called Documents by Readdle. Installed it on my iphone, sync'd it with my Dropbox account. Clicked on Search then tapped "contents". It then asked me if I was sure because it would need to index the files. Once I said yes it ran through the files and I was able to search them.