Updated 4/5/2019 There are tons of these apps. I’ve only tried a few. Many of these apps can open and annotate pdf files also. I’ve tried a few others but these stand out. My current favorites are Noteshelf and Goodnotes.
Updated 4/5/2019 While I’m not interested in reviewing apps, I’ve been asked several times which ones I use. This is my step to answer those questions. Keep in mind that word processors offer more formatting options than text editors.
Updated 4/5/2019 I spend a lot of time reading articles for research–these articles are generally pdf files. I’ve tried several different annotation packages for the iPad. Note: I have no interest in reviewing apps–lots of other people do that already. However several people have asked what apps I use. Your preferences will vary.