Comment by 3036e4

I kind of agree and see what you mean, but what I described happen often that I forget where I came from and have no idea that the PDF I read was opened from some other app.

It's part of what I mentioned in another comment, that BACK button can feel random. "Did I open this PDF from within the PDF viewer or from some other app? What app?" Instead of the BACK button having a predictable, known, function, it depends on some hidden state.