Notability (iOS)
Used by 7 peopleapps.apple.com ↗

“(I'm "webgoddess" over there if you want to friend me.) I use Notability on my iPad to organise my knitting patterns and mark up PDFs with the Pencil.”
Kris Howard Interview, Tech leader, blogger, crafter, speaker · via Uses This 
“Note-taking, writing, blogging, conference speaking: – I LOVE Notability – being able to write using the pencil with my iPad, but also add images, type text, draw, highlight, all in one app, still amazes me, and fits my writing style very well.”
Lena Reinhard Interview, Director of Engineering, CircleCI · via Uses This 