Visual PDF Comparison

A visual comparison renders each page of both documents to a bitmap at matching dimensions, then compares them pixel by pixel with a small perceptual tolerance so that anti-aliasing differences do not register as changes. What it catches is everything a text diff misses: a logo swapped, an image moved, a colour changed, a table re-ruled, a font substituted, a signature block shifted. It is also the only method available for scanned documents, which have no text to compare. Its central weakness is worth understanding first: pixel comparison has no idea that content can move for innocent reasons. Insert one word on page two and every line below it shifts down, so from there on every page differs pixel-wise while reading identically. That is why the report gives you a difference percentage per page rather than a verdict — a page with two per cent changed pixels in one region is a real edit, while a page showing thirty per cent uniformly shifted is reflow.