I would like to share my experience with using the iPad, from the original version through my current iPad Pro insofar as good apps go. I mean, of course, in addition to Goodreader---which someone once called the "swiss army knife" of iPad apps! I use Goodnotes every day to do handwritten math and other work related to my research on electromagnetic field theory. I have found that it works very well with the Apple Pencil, though there is a current glitch in pasting using the pencil. (Steve, the developer, says that an update is imminent.) I know that the subject of handwriting apps is highly ideosyncratic, but Goodnotes is certainly "good." For doing illustrations of publication quality, I have found that Inkpad is nothing short of great. It is easy to learn, nothing at all like autodesk or the like. Of course if you are a mechanical engineer or architect you will probably find it confining. But for the kind of things we electrical engineers do it works fine. For writing technical papers, teX is one of the most ubiquitously used text editors---and Tex Writer is an excellent implementation for the iPad. I gave it five star rating in the app store when it appeared and have no reason to regret it. There are several other teX apps available now. I tried them all early on and liked Tex Writer the best, though that opinion is a couple of years old. I only wish my Apple keyboard had the teX symbols more easily accessible. Since it does not, I use the virtual version supplied by the app. As an afterthought, I should mention Tex Equation, which is useful for writing math to be imported into Inkpad to place on drawings created there. That's all I can think of now, but I hope others will chip in and add their own evaluations for their special interests. All the best!