Many free TTS programs give you the option to download an audio file of the speech to save and transfer to different devices.
MP3 is the most common audio format, and compatible with pretty much any modern device capable of playing back audio. The WAV format is also highly compatible too.
The main difference between the two is quality. WAV files are uncompressed, meaning fidelity is preserved as best as possible, at the cost of being considerably larger in size than MP3 files, which do compress.
Ultimately, however, MP3 files with a bit rate of 256 kbps and above should more than suffice, and you'll struggle to tell the difference when it comes to speech audio between them and WAV files.