In many books, particularly textbooks, the author or editor will say "see above" or "see below" in reference to earlier or later pages, respectively, even though pages aren't arranged in a way that this makes immediate sense. Is this a consequence of digital word processors, where (most often) the pages appear in order from top to bottom, where earlier pages really are above and later pages really below? Or is this an older idiom, and, if so, where did it come from?
Not even a little bit. Writers were using the phrase long before computers were even remotely conceived of. In English the phrase has pretty much always been in use, despite the use of the codex in almost all English publication since the creation of a written English language--it simply refers to the relative position on the page. The phrase goes back at least to the classical authors--Sallust uses some variation of the phrase at least four times in the bellum Catilinae alone