PDF works well on a computer, and not so well on smaller devices.
ePub works well on iPhone and Android.
Mobi may be needed on some devices that will not take ePub.
iPhone: An eBook reader should be on the Apple products by default.
Android: These products may need to have one installed. One app is FBReader. The FBReader TTS app (talk to speech) can be added to read the eBook out loud.
On Android, you may be able to download the ePub files and copy to a folder on the device or to a Sim card to install in the device.
Windows PC: eBook readers are available as well. For Windows 10, check the App Store. For older versions an eBook program may be available.