GoodNotes - GoodNotes lets you take notes and annotate PDF documents.
Sketch Mirror - Preview your iOS designs directly on your devices
Milanote - Milanote is a note taking app for creative work.
Crystal: Sketch Mirror for Android - Crystal: Sketch Mirror for Android is an application that is designed for viewing your sketch designs in real-time, navigate prototypes and download content related to the sketches for offline viewing.
OneNote - Get the OneNote app for free on your tablet, phone, and computer, so you can capture your ideas and to-do lists in one place wherever you are. Or try OneNote with Office for free.
Flowkit - Sketch library for user flows/content maps/annotations