Image: Gingerlabs.com (Developer)
I've tried a few apps to annotate sides and other PDFs on my iPad, including the old favourite Evernote. I use an iPad Mini, and would like to upgrade to an iPad Pro in the future. The limitations I've had with other PDF annotation apps I've tried usually start with sloppy recognition of the stylus. This resulted in difficult or slow writing unless the line thickness was unreasonable on the Mini. The best I've used on the Mini (first discovered on an iPad Pro I worked very hard not to buy on credit) is Notability by Ginger Labs. It's priced at $13.99 CND at the time of writing. It's very easy to use. The Mini easily sends a copy of a PDF script to Notability for marking up. It responds quickly and accurately, both for writing and highlighting. The pen strokes are smooth - I've even used it for digital signature on an NDA and it looked beautiful and smooth, even with a stylus that came on a pen from the vendor's hall at a dental convention, rather than a high end one (or the Apple Pencil). It took a few tries to get used to it, though. What I really like for voiceover in this app is that you can have it record a sound clip for you to embed on bedazzled PDF. Great if you have some ideas for voices to use, that you want to save for later. Easily integrates to email (PDF export) and other export options.
My only grievance is not Notability's fault. When I write on my PDFs, my writing is a little on the large side, because the Mini's screen is so small. You can work around it by zooming in where you want to write before writing, but that is annoying. I've used this app on the 12.9" iPad Pro and it was absolutely glorious and not a problem at all. Verdict: It's the app for me!