How do I change the font size in Apple Books?

Tap the aA icon in the top-right corner while reading. Drag the font size slider left (smaller) or right (larger). Tap the A buttons for quick increments. You can also change the font typeface, background color (white/sepia/dark), and line spacing from the same menu. Changes apply immediately and sync across iPhone, iPad, and Mac.

Can I change the font on a Kindle EPUB?

Yes. While reading, tap the top of the screen to show the toolbar, then tap Aa (Display Settings). Choose from Kindle's built-in fonts including Bookerly (Kindle's custom serif), Amazon Ember, Caecilia, Georgia, and others. Adjust font size, line spacing, margins, and page color. Publisher fonts in the EPUB may override these settings — tap Font Override to force your choice.

Why can't I change the font size on a PDF?

PDFs are fixed-layout documents — the text is positioned at exact pixel coordinates and does not reflow. Zooming in on a PDF doesn't change the font size; it just magnifies the fixed layout, requiring horizontal scrolling. EPUB text reflows to your screen and font size settings. To get adjustable fonts from a PDF, convert it to EPUB using toolkit.bot/pdf2epub.

How do I change font settings in Kobo?

While reading, tap the center of the screen, then tap Aa (Typography). Adjust font size with the slider, choose a typeface (Kobo Nickel, Georgia, Palatino, and others), set line spacing, and adjust margins. Kobo also lets you enable side-loaded fonts by placing .ttf or .otf files in the fonts/ folder on the device via USB.

How to Change Font, Size, and Spacing in Your EPUB Reader (All Apps)

One of the biggest advantages of EPUB over PDF is that you can adjust how text looks — font size, typeface, line spacing, margins, and background colour. Every major EPUB reader supports this. Here's how to do it in each one.

Apple Books (iPhone, iPad, Mac)

Open an EPUB in Apple Books
Tap the aA button at the top of the screen
Adjust font size with the small/large A buttons
Tap Font to choose from Georgia, Palatino, Athelas, Seravek, Charter, San Francisco, or a custom installed font
Tap Themes to switch between White, Sepia, Gray, or Night (dark) mode
Use the sliders for brightness, line spacing, and margin width

Apple Books applies your settings to all EPUB books in your library, so you set it once and forget it.

Kindle app (iOS, Android, Fire tablet)

Open a book in the Kindle app
Tap the centre of the screen to show controls
Tap the aA icon (top right)
The Font tab lets you: choose from 8 typefaces (Bookerly is Amazon's custom book font), adjust font size, bold weight, and character spacing
The Page tab controls: line spacing, margin width, orientation lock, and layout
The Color tab: White, Sepia, Green, Dark, or Blue backgrounds

Kindle's Bookerly font is specifically designed for reading on screens — worth trying if you haven't.

Kobo e-reader

Open a book on your Kobo
Tap the centre of the screen → tap the font icon (Aa) at the bottom
Adjust font size, line spacing, margins, and brightness
Tap Font to pick a typeface — Kobo offers 10+ including Merriweather, Georgia, and Gill Sans
Turn on Publisher defaults to use the font the publisher chose; turn it off to apply your own settings

Thorium Reader (Windows, Mac, Linux)

Open a book in Thorium
Click the settings gear icon at the top right
Choose from: font family, font size, letter spacing, word spacing, line height, paragraph spacing, column count
Colour mode: Default, Sepia, Night, or a custom foreground/background colour
Thorium has one of the most detailed typography controls of any EPUB reader

Calibre viewer (all platforms)

Open an EPUB in Calibre's built-in viewer (double-click a book in your library)
Click the settings gear icon → Preferences
Adjust font size, line height, margins, and font family
You can also inject custom CSS — useful for developers and power users who want precise control

Why can't I change font size in a PDF?

PDF is a fixed-layout format — every character is positioned at exact pixel coordinates on a fixed page. There is no text flow to reformat, so font size controls don't exist. You can zoom in, but that just scales the entire fixed page.

EPUB uses HTML and CSS internally — the text is marked up like a web page, and the reading system renders it at whatever size you choose. This is why EPUB readers can offer all these typographic controls while PDF readers cannot.

If you have a PDF that you want to read with adjustable fonts, convert it to EPUB first:

Go to toolkit.bot/pdf2epub
Upload your PDF — conversion takes 15–60 seconds
Download the EPUB and open it in any reader above

You'll immediately get font size control, dark mode, and comfortable text reflowing to your screen size.

Convert your PDF to EPUB — then set any font, size, or spacing you want.

Convert PDF to EPUB →