I am looking for an iOS app that will convert a text to an audio file that I can then listen to in that app or export it to another audio-player app. So the app should not just read out the text, but give the option to convert to audio file (like mp3).
There are several apps to do this. One is called iSpeech which is highly rated. It may work only on text files but i'm not sure of that. Another popular app is ClaroPDF which is specifically for PDF files. It will read the files and save the reading as a sound file.
A good article rating the top text to speech apps is found here