How to compare two PDFs

How to compare two PDFs, step by step

Comparing two PDFs means seeing what changed between two versions of the same document. Three steps, nothing to install:

1. Open the Compare PDF tool.
2. Drag both files into the window. The first is the reference version, the second is the one being compared.
3. See the differences on screen: the first pages appear side by side straight away, with the differing areas marked on both sides.
4. Download the report, a PDF with every compared page, one sheet per page, with the differences painted over.

All the processing runs in your browser, start to finish. No account, no upload wait, and neither PDF is sent to a server to be compared.

Visual comparison: what counts as a difference

The tool compares the image, not the text. It draws both pages and measures where the pixels diverge: it does not read words, and it cannot tell a sentence from a logo, it only knows that area of the sheet stopped looking the same.

That has two practical consequences. Text that merely moved counts as a difference: adding a paragraph at the top pushes everything below it down and the lines no longer line up with the other version, the words are the same, the position is not. And a change that does not alter the page appearance (metadata, the author, hidden layers, an annotation that is not drawn) does not show up at all; to see what the file holds inside, use PDF properties.

Both files do not need the same number of pages: pages in common are compared, and pages that exist in only one of the documents are identified by number, in the summary and in the final file. Pages of different sizes are reduced to a common scale before any comparison, each keeping its own proportions, two exports of the same document at different resolutions come out as identical, which is what makes sense.

Tips for comparing PDFs

• Make sure which version is the reference. The first file you drag in is the starting point; the second is compared against it. In the report, every page shows both versions side by side, with the differences painted over both.
• Areas marked where you did not touch anything? Lower the sensitivity. On high sensitivity, tiny drawing differences, like edge smoothing between two exports, are enough to light a block up. Normal sensitivity ignores them.
• Know the page limits. The preview stops at the first twenty pages, because each one costs two renders, but the file you download carries the whole document, up to the tool limit. And a document with too many pages is refused before even trying, by a safety limit: the message states the limit, split the file first.
• Changes that do not alter the appearance are not detected. Metadata, the author, hidden layers, an annotation that is not drawn, a font swapped for one that draws the same: none of this moves a single pixel. To see what the file holds inside, use PDF properties.
• After comparing, deal with the result. Found a sensitive area in a contract you are about to share? Redact it with Redact PDF. And if the two versions you want to compare are spread across several files, merge them first with Merge PDF.

Is comparing PDFs free? What are the limits?

No. This tool is part of the Premium subscription, EUR 1.99 a month, which removes the limits on every tool and adds OCR. Sign PDF opens on the free plan, with one signature a day.

Sources

• BestPDFInTheWorld, Comparar PDF
• MDN, File API
• PDF.js