Apple does not allow MP3 downloads from outside companies to be downloaded and added to the music app. Therefore, downloading to iPhone or iPad is a two-step process:
Download the ‘Documents: Files, PDF, Browser' app, launch it and then download the file via the app. Please watch the step by step video tutorial in the video below:
Step 1: Download the files to your computer.
Step 2: Transfer the files to your iPhone/iPad via iTunes.