Some platforms allow you to choose the download format. The two most common formats are:
PDF:
Most devices can read a PDF file, but it may be difficult to read on smaller screens.
ePUB:
Text flow is adjusted to fit the screen of your device (this may change the pagination). This format is used by Adobe Digital Editions and can be read on most devices including Apple and Android. an open-standard file format for digital books and one of the most widely supported along with .pdf and .txt. EPUB eBooks also support embedded content, allowing, for instance, a Flash video to be embedded within an ebook.