I received my DX a week ago now and am extremely pleased with it. PDF support is just fine - no zooming, no linking, no embedded video or HAL 9000 conversations either. It's fine for the PDFs you think it's gonna be fine for and likely not as fine for the ones you're suspicious about. Yes, symbolic stuff like math notation displays just fine - just not at microfilm resolution.
No, it won't zoom much - get a notebook if you want that. It's essentially a sheet-of-paper-emulator that the media has mistaken for a notebook. So if you'd expect to have to squint or hold the paper closer to your face on an economy-mode print of that PDF, expect the DX to struggle with it. All very common sense IMO.
This is a book reader that was made in the year 2009, not a holographic tricorder from 2020. It's primarily aimed at people who are satisfied with the written word in quantities exceeding the size of your typical AP story and who can sit quietly and just read for more than 30 minutes at a shot. Every dissatisfied Kindle review I've read tries to interpret the device in terms of the writer's politics (looking at you, DRM nuts) or some flashier gadget category, which has just reinforced my suspicion that reviewers are ADD children who just don't do much reading.