I needed one thing: keep two PDFs open side by side and have them scroll together. The Italian edition of my book on the left, the English one on the right, and the ability to move down the text without losing the alignment.

It sounds like a feature that has always existed. It doesn’t.

I spent an evening looking for it the way you look for a library before writing your own, which is to say expecting to find it and impatient to get back to the real work. The first place I checked is the viewer I use daily, SumatraPDF, which is fast, light and well built. Side-by-side view was first requested there in 2011, then again, and again after that, in three separate issues over ten years. All were declined, and the maintainer’s position is consistent and legitimate: that program intends to stay what it is.

The second place is a widely used commercial PDF editor, where the same request has been open since 2017 and has never produced a native feature. The third is Acrobat, which can put two windows side by side the way any window manager can, but has no scroll lock between two independent documents.

The only program that genuinely did the thing was an editor called PDF Architect, and I used it long enough to understand why it wasn’t enough. It couples the two views on fixed page pairs, one with one, two with two, and to hold them together it follows the raw scroll position. As long as the two documents are identical, it works. Mine aren’t: the English edition runs about ten pages longer than the Italian, because English takes up different space and a few chapters stretched. After thirty pages the two views are showing two different parts of the book, and you’re scrolling two disconnected documents with the distinct feeling of working against your tool.

At that point I did the calculation we all do. On one side, a feature requested without success for fifteen years, which usually means it’s harder than it looks or that not enough people care. On the other, a need of my own, daily, containing a well-defined and rather appetising technical problem: the synchronisation must not be done on scroll pixels, it has to be done in page units, otherwise two documents with different page sizes diverge by construction.

On 10 July I made the first commit.

I know this is the most overused story in our trade, the one about the person who writes their own tool instead of adapting to what exists. Usually it’s a bad idea too, because the time you sink into it never comes back. The difference here is that the need wasn’t a configuration whim: it’s how I work on three editions of the same book every single day, and nothing on the market covered it.

Fifteen years of declined requests don’t tell you the feature isn’t needed. They only tell you it wasn’t needed by the programs it was asked of.

Last modified on 2026-07-10