Quote from the book "Уровни сложности (Читер 4)"

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

Accessible from nearly any device

Works on any computer

Works 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