Quote from the book "PHP. Разработка модуля комментариев для сайта"

Suitable for smartphones, Android tablets, e-readers (except Kindle) and many programs

Can be opened on almost any device

Can be opened on any computer

Can be opened on any computer

Opens in Adobe Reader

Optimized and suitable for smartphones

Suitable for Kindle e-readers and Android apps

Suitable for iOS devices (iPhone, iPad, iMac) and most reading apps

Perfect for iPhone and iPad

Development of FB2 format