There are a lot of classic PDF features that are still unimplemented by alternative viewers. For work I have to use Reader for clickable metadata popups that other viewers don't support.

There are two big features that Adobe supports that I just don't see common in other readers. First, the schematic capture application I use will generate a PDF of a schematic that has metadata of each component accessible by clicking it. The schematic will show "R179, 100ohms, 0.125W" but clicking to see the metadata will show a part number along with whatever other data the BOM has. No other reader I've used will show this data. Since the schematic PDF serves as our "immutable" copy that goes into our CRM system, it's nice that you have everything you need without having to open Cadence. I believe this is some sort of JavaScript extension to PDF that is likely incredibly exploitable so this is likely why no one else seems to support it (and why Adobe always seems subject to CVEs).

Second, we use Adobe's comments to markup released drawings or other documents for changes. Then both I and QA put our signatures on the PDF and it's either sent to the factory floor for immediate implementation or sent to the document owner for them to incorporate into a new release. Other readers don't always use comments the same way or don't respect the read only attribute that comments and signatures should have.