New
#200
I believe the in-built Reader app has been taken out as at build 10122 (or possibly internal-only 10120). The idea is you're supposed to use Edge instead of Reader or Reading List, although I did see somewhere that you could install Reader from the Store.
From here:
Note that the Windows Reader is a totally separate thing to the Adobe Reader (or whatever it's called now).Although the Reader and Reading List apps both ship with build 10122, if you click on a PDF file, it’s opened in Edge. According to the build 10120 Breaking Changes list, it looks like both Reader and Reading List are being deprecated.