Ask HN: PDF-based notetaking apps for iPad Pro and Pencil?

Paper notebooks have long been an essential part of my software development workflow, and now I'd like to take them digital with the iPad Pro and the Apple Pencil. Unfortunately, it seems that most of the notetaking apps on iOS are fairly proprietary: your sketches are stored in some hidden database or in the cloud, and the only way to get something standard out of them is to perform a clunky export. I'd like my notes to be in a standard format, and I'd like them to be editable directly; in other words, there should always be a 1:1 mapping between the files sitting in my Dropbox and the notes that the app is editing. So I was wondering: are there any Pencil-supporting iOS apps that simply let you create a new, blank PDF — maybe even with a custom cover image — and then use it directly for sketching and perhaps even text and image entry? Huge bonus points for handwriting recognition!