Where I am, it's perfectly legal.
Before cell service was as widespread as it is today, there were programs that would scrape web pages into ePUBs so you could read them later on your Palm Pilot. I used it every day during my commute. And the best part was that they ended. No mind-numbing infinite scroll.
When I switched to a "smart" phone (SonyEricsson m600c), I really missed it.
I spend enough time at a computer than I shouldn't really need a smartphone outside of 'I need to message ___' or 'I need to go ___'
https://medium.com/@chrisdesalvo/the-future-that-everyone-fo...
If you start selling the resulting files, now that would be a copyright violation. German law has a right to create a "Privatkopie", i.e. a private copy. I guess this is similar to fair use in US law?