If you’re reading in an e-book reader such as a Kindle, Calibre or iBooks you should look for the type controls that often look like AAA or sometimes just A. You can experiment there and set your preferred size and font as the default for all books if you wish. The same area may let you change the colour of the page and text. Try out the different combinations on offer: some of them are easier on the eye allowing long-term reading with less eye fatigue.
With PDFs in Adobe Reader or Preview you don’t have the same control. You can use the magnifying glass to make the page bigger or smaller, but this doesn’t change the relationship between the size of the type and the length of the line. But larger type is more legible, up to a point, so it’s worth experimenting to improve legibility.
Some e-book reading devices offer layout controls so that you can view one page in portrait (upright) orientation or two in landscape (sideways). Choose whichever suits you best.
It’s worth experimenting with reading ebooks on your smartphone. Conventional wisdom suggests that the smaller the screen the harder it is to read. But a combination of large type and a small screen requires almost no lateral movement of the eyes and some people find it easier and quicker than the alternatives (at least until thumb fatigue sets in…). There is no question, however, that it’s much harder to make annotations on a phone screen. Don’t bother trying to read large PDFs on your phone as the page won’t reflow to accommodate the small screen.