Why reading two PDFs side by side doesn't actually work
A contract comes back after a round of edits, and you're told "just a couple of small changes." Reading both versions side by side to find them sounds reasonable until you're twelve pages in and starting to lose confidence you'd notice a single changed number or a quietly removed clause. Human attention isn't built for finding a needle in an otherwise-identical haystack — it's built to skim for meaning, which is exactly the mode that lets small, deliberate or accidental changes slip past.
A pixel-level comparison tool doesn't get tired or skim. It renders both versions and flags every place they differ, mechanically, page by page.
What the red highlighting is actually telling you
Both PDFs get rendered to images and compared pixel by pixel — anywhere the color difference between the two crosses a threshold gets marked in red, while unchanged regions fade to grayscale so the red genuinely pops out. This means it catches everything: a changed number, a moved paragraph, a font swap, a logo update, a clause that shifted position because something above it got longer.
It's worth knowing this is visual, not textual — it flags where pixels changed, not what a word-level diff would call "added" or "removed" text. Identical text re-rendered at a slightly shifted position can register as a difference too, which is expected behavior, not a false alarm to ignore blindly.
Using the diff percentage to triage, not just to look at pretty red pages
On a long document, the real value isn't staring at every page — it's the per-page diff percentage that tells you where to actually spend attention. A page sitting at 0% needs no review at all; a page at 15% has a real, substantial change worth reading carefully. This turns a 40-page "did anything change" review into a 3-page targeted read, which is the entire point.
Trace-level differences (under roughly 1%) are usually just rendering noise — subtle anti-aliasing differences between two rendering passes — rather than a genuine content change, so don't panic over every nonzero number.