Marcin Borkowski (mbork) has a useful post on scrolling PDFs in the other window. He wants to use the familiar Ctrl+Meta+v and Ctrl+Meta+Shift+v shortcuts for pdf-tools. Sadly, these don’t work for scrolling PDFs but as mbork says, “… [I]t’s Emacs, so it shouldn’t be difficult…”. And, indeed, it isn’t.
His idea is simple. If the buffer in the other window isn’t a PDF, return nil. Otherwise simply call pdf-view-scroll-up-or-next-page or pdf-view-scroll-down-or-previous-page. Finally, he simply advises scroll-other-window and scroll-other-window-down to run his code first and skip the normal code unless his code returns nil. Take a look at his post for the code. As I said, it’s easy to understand.
The important takeaway from this, according to mbork, is how easy and understandable the solution is. The lesson, as it often is, is that if Emacs isn’t doing what you need, it’s usually pretty easy to make it do so.