Although I use markdown (and similar) for memos, I turn to latex for longer and more complex material.
A lot of this is just because latex has been a standard for publishers in my field since I started (approximately a thousand years ago).
When writing for journals, latex saves a lot of work. Publishers provide latex templates that ensure that articles have a prescribed format and scope of content. Being able to see a good facsimile of the final published form is quite handy for authors. Oh, this paragraph is going on for over a column -- I'll break it up. That sort of thing.
This still applies when writing for longer things, such as textbooks and course notes, but another factor (for me, the larger one) is that latex (more properly, the tex upon which latex sits) is a programming language. Macros can be written to do lots of things that would be a pain if done manually, and once a macro is written, altering an entire text is easy. I did this in a book I wrote a while back, writing macros to colourize text that would be indexed, add margin notes for things I wanted to return to, categorize paragraphs by function, and so on. I could turn all those macros on and off by uncommenting a line. This is really quite helpful in writing something that takes months to years to complete. Frankly, I use this macro approach even in memos written in markdown. Inside almost all of my markdown documents, there are latex commands.
As for reading things on a small screen, which I guess is really the topic here, I must admit that this is something I rarely do within my own field. Sure, I do it if reading one of those 10-km overview articles in Science or Nature. But when it comes to my own field, things are technical and demand long periods of study. I don't try to read this stuff on the bus or in a coffee queue. I need time (hours or days) and I need to be able to take notes.
Another reason I prefer PDF is that it is fixed. My brain puts information into a sort of spatial framework. Somehow, if I look at a paper I first read 40 years ago, I still know what information is on which page, which of the diagrams summarizes the whole thing, and which of the citations is key. This may be a flaw in my brain functioning, but I just don't find these sorts of memories forming when I read content that has a plastic format, with paragraph breaks changing if I adjust my view. But maybe this is just my age talking, I suppose.