Whio

Read technical books comfortably.

Whio is an EPUB and PDF reader for technical books on iPhone and iPad, with a focus on readability and accessibility.

Whio on iPad with a paper-colored theme and a serif font, showing two pages of A World of Characters with a table and an illustration of an emoji sequence.

Whio on a partially folded iPhone Duo in a dark theme, showing Swift Charts Beyond the Basics with highlighted Swift code beside a temperature bar chart.

Whio on iPhone in a dark theme, showing a tip from Swift Gems with a highlighted Swift code example.

Read technical EPUBs and PDFs in an app built for them.

Whio is designed specifically for technical books on iPhone and iPad, including iPhone Duo. With support for code examples, diagrams, and detailed explanations, it gives you clear syntax highlighting, illustrations to explore, and independent reading preferences for text and code.

Whio on iPhone Duo in portrait, showing A Taste of Swift with an illustration of enumeration cases and a table.

Whio on iPhone Duo in landscape with a dark theme, showing Paths to a Solution with an illustration of a hash table.

See code clearly and try examples yourself.

Follow the structure of code with syntax highlighting and preserved indentation. Scroll horizontally through long lines, or zoom an individual example in or out without changing the surrounding text. Copy code directly from the book when you're ready to try an example yourself.

Whio on iPhone in a dark theme, showing a Python binary search function from Paths to a Solution with zoom and copy controls.

Customize typography independently for text and code.

Choose fonts that are comfortable to read, including options with distinctive letter shapes, and adjust text size, weight, and line spacing to your preference. Code has its own font, size, and weight settings, so you can make examples larger or bolder while keeping the prose as you like it.

Whio text preferences on an open iPhone Duo in light appearance, showing the Atkinson Hyperlegible Next font, medium weight, 120 percent font size, 150 percent line spacing, and a preview of the Purple theme.

Whio code preferences on an open iPhone Duo in dark appearance, showing the JetBrains Mono font, semibold weight, 110 percent font size, and a preview of highlighted code in the Dracula theme.

Choose themes that suit your reading needs.

Whio offers high-contrast themes, softer contrast, and tinted backgrounds for different reading needs. Select themes independently for text and code, whether you prefer a softly tinted page, more contrast in examples, or high contrast throughout. Each theme includes light and dark variants, with the option to follow your device's appearance.

Whio on iPhone with a purple tinted theme for both text and code, showing A Taste of Swift.

Whio on iPhone with a high-contrast dark theme, showing a section of Swift Charts Beyond the Basics with a chart of sleep sessions.

Get additional support as you read.

Whio offers optional reading aids for giving letters more definition and keeping your place in the text.

Give letters more definition.

Letter distinction adds space between letters that can blend together and subtly varies their emphasis to help distinguish letter shapes and word boundaries. Adjust the intensity to find a presentation that suits you.

Whio reading aids preferences on iPhone, showing letter distinction turned on at 70 percent intensity with a preview of the adjusted letter spacing and emphasis.

Whio on iPhone with a peach tinted theme, larger text, and letter distinction applied to a page of A World of Characters.

Keep your place as you read.

A reading guide lets you underline the line you're following or keep a few lines in focus while dimming the surrounding text. Move the guide through the passage as you read. With the focus window, you can choose how many lines stay in focus and how much the surrounding text is dimmed.

Whio on a partially folded iPhone Duo in a dark theme, with the reading guide keeping three lines of The SwiftUI Way in focus and dimming the rest of the page.

Whio reading guide preferences on a closed iPhone Duo, showing the focus window style with three lines and 50 percent intensity.

Save passages and notes in your Notebook.

Highlight a passage or a code example and add a note of your own, or bookmark a page to come back to. The Notebook collects everything you save across your books, so you can search it, open any passage in its book, and export a book's highlights and notes.

Whio on iPad, showing a highlighted passage in Swift Charts Beyond the Basics with its saved note.

Whio is 100% indie made, created by Natalia Panferova, a former Apple engineer, Apple Entrepreneur Camp alumna, and technical book author based in New Zealand.

Download Whio for iPhone and iPad.

Works with DRM-free EPUBs and PDFs.

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