You're absolutely right PDFs are super rigid, but that's kinda their point - so with the proper device, like a sheet of paper or a 10+ inch tablet screen it makes sense.
Would I prefer more content to be reflowable etc.? Yes - but with a tablet it isn't strictly necessary, just nice to have.
It's plenty versatile. Not everything needs to be phone-friendly. Phone screens weren't designed for reading PDF-size documents. Even so, options exist to reflow the text, view in landscape or pan and zoom.
Would I prefer more content to be reflowable etc.? Yes - but with a tablet it isn't strictly necessary, just nice to have.