When downloading a Media (PDF, MOV or MP3) File, your files will go directly to your "Downloads" App.
Unfortunately, iPhones don't allow downloading MP3's or other types of media files. We wish they did but they don't. If you are using an Apple device, you will want to take a moment to check out these alternatives if you wish to download a Media File to your mobile device for storage (iPhone or iPad) and use at a later time.
The easiest way to download a file is to do so from your computer and drag the file to your iCloud (for PDF) or iTunes (for MOV or MP3) Files. When your iPhone or iPad has synced with your iCloud and iTunes, it will then be available to you on your phone.
However, if you are downloading the file directly to your iPhone or iPad, we have a workaround available to you.
We also hear great reviews about Documents by Readdle which can be downloaded here. (We are not an affiliate, and can't vouch for them outside of reviews we have read suggesting them and any help with this is out of our scope in Support)
To access your file when offline, you will need to edit your settings in the application account.
Please view those directions pertaining to your application here:
Google Drive here Or for Dropbox here.