On an iPhone that supports 3D Touch applicable to iPhone 6s or iPhone 7 press firmly on the preview to make it expand and fill the screen. On an older iPhone without a pressure-sensitive display, perform a pinch-out gesture instead. This means placing two fingers on the page thumbnails, and spreading your fingers apart from each other.

Source: How to print to PDF on iPhone, iPad, and Mac - 9to5Mac

This is awkward compared to Android (especially on the iOS Simulator), but hey, it exists.