You can increase the size of the text that you read to make it more legible, or decrease it to fit more onto your screen.

If you are viewing a PDF-based document (or a non-reflowable textbook), click the four-arrows icon in the toolbar to view options for zooming the page:

If you are viewing a EPUB- or web-based document (or a reflowable textbook), click the + or - icons in the toolbar to increase or decrease the text size: