If you do your pages vertically, imagine if you could add notes on a horizontally added page. This would be significantly helpful for PDFs which take up the entire page for each page so you can create summaries, pointers or organize what is in the particular page.

On a separate note, it would be a nice addition to have this horizontal page be floating and following you as you scroll so you can continue to update this one page to potentially summarize the entire document in that one page (this could be an alternative option to adding a single fixed page on the horizontal plane of choice)

NOTE: If you do your pages horizontally, the concept would be to add pages vertically for extra space for notes.