The app will try to make simple text-to-speech and speech-to-text conversions, if you attempt conversions like MP3 to PDF or PDF to MP3, etc. This will usually only work for simple files in English. The app is primarily for converting documents to other document types, audio to other audio types, etc.

For more complex tasks,  you will want to use a specialized app for that purpose:

If you are trying to convert a complex document to speech, try Natural Reader: https://apps.apple.com/us/app/naturalreader-text-to-speech/id598798210 

If you are trying to transcribe (speech to text) and audio/video file, try the Transcribe app: https://apps.apple.com/us/app/transcribe-speech-to-text/id1241342461