How to Compare Two PDFs and Find What Changed

Someone returns a contract "with a few small changes". Reading forty pages twice to find them is a poor use of an afternoon, and it is exactly the kind of task where the eye reliably skips the one change that matters.

Compare two files

1. 1Open the Compare PDF tool and add both versions.
2. 2Put them in order — the original first, the revised second.
3. 3Run the comparison. Differences are reported page by page.
4. 4Review each one and decide whether it was expected.

Two kinds of comparison

Text comparison is the one you want for contracts. Visual comparison is better for design work, where a moved logo matters as much as a changed word.

What comparison cannot tell you

• Whether a change was authorised, or merely present.
• Which version is the one that was agreed.
• What changed in a scan, unless both files have been through OCR.
• Whether an apparent difference is just a reflow after a font substitution.

That last one causes a lot of noise. If a font was substituted, the text wraps differently and the comparison reports differences on every page. Convert both files consistently before comparing, or use a text-based comparison that ignores position.

Comparing scans

Two scans of the same document are never pixel-identical — the paper sat differently, the exposure varied. Visual comparison reports differences everywhere. Run OCR on both and compare the text instead, accepting that recognition errors will produce some false positives of their own.

Doing it properly on an important document

1. 1Compare the text and read every difference.
2. 2Check the page count and the section numbering.
3. 3Check the schedules and annexes, which is where changes hide.
4. 4Confirm the signature blocks and dates are unchanged.
5. 5Ask the other side to summarise their changes, and check that against what you found.