How to view the same file in different apps in iOS? I'm using an iPhone for the first time and I found that I can't view the same file in different apps.

I have have some PDF files added to Adobe Reader but I cant view them in a similar app that views PDF files. Same thing happens with songs, photos and videos.

Does it mean that I have to add the same files to all other apps again? If so, wouldn't that be duplicating the same files over and over again? 

Is there a way to add the files once and use it in all apps like in Android?
Yes, you will need to add the file to all the apps. You should be able to use the system-implemented Open With option, but this requires the support of the app to transfer the file.

Due to the iOS sandbox, the apps aren't able to access the files from another app — the file must be duplicated to the second app.