[enriquto]

> It's better than PDF. Text in PDF is not reflowable.

This is relative. Many people dislike reflowable text, and often it is not very convenient. Book typesetting is hard, and once it is done, reflowing the text changes the book, usually for worse. Maybe not for plain text, but definitely for math and poetry, were the "flow" is a crucial part of the work, and it was carefully set up by the author. You do not ever want to "reflow" math or poetry. For these two subjects (my favorite ones, incidentally) pdf seems to be the best option.