Translation:Everybody is reading the same newspaper.
These sentences are hard to translate into English, but in this case, these people would, quite literally, be "walking and reading," because that's what written here. If they were sitting and reading, certainly, one would say "sidder og læser," even if both ultimately get translated the same way into English.
(These kinds of constructions are in Dutch too, so I've become quite accustomed to the logic behind these!)
This construct just is used to build the Gerund in Danish. It doesn't make a whole lot of sense to translate it word for word. https://www.duolingo.com/skill/da/Verbs%3A-Gerund/tips-and-notes