Inter

Inter is a free and open-source neo-grotesque sans-serif typeface designed by Rasmus Andersson. Released in 2017, Inter was created to improve on-screen readability for user interface text, especially at small sizes. The typeface has since become widely used in applications and websites. Inter includes a wide range of OpenType features, support for over 140 languages, and is available as a variable font.

From the Wikipedia article “Inter (typeface)”, licensed under CC BY-SA 4.0.

Sans-serif faces work well for headings, captions, children's books and modern nonfiction. For long-form body text in print, most designers still prefer a serif.

Commercial typefaces require a licence that covers embedding in PDFs and ebooks; check the foundry's licence before you publish.