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.

How to find differences between two PDF 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. 4Open the flagged pages in both files side by side, and decide whether each change was expected.

What the comparison shows you

Slate PDF reads the text of every page in both files and compares them page against page. You get a verdict — identical or not — and a list of the pages whose text differs, plus the page count, file size and document properties (title, author, creating application) of each file side by side. It points you to the pages that changed; it does not highlight the individual words, so the reading is still yours to do.

The verdict ignores the filename and the exact byte size. A file that was renamed or re-saved without changing its content still comes out identical, which is usually the reassurance people are after.

Two kinds of comparison

Text comparison is the one you want for contracts, and it is what the Compare PDF tool does. Visual comparison is better for design work, where a moved logo matters as much as a changed word — a change of colour or position that leaves the text alone will not be flagged by a text comparison.

When to compare two versions

• A contract changed between the draft you reviewed and the final you were sent.
• A colleague edited a report and you want to confirm nothing went in besides the announced changes.
• A long document was regenerated and you need to know which pages were actually touched.
• Just before you sign or send: compare the file in front of you against the version you approved.

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.
• Which pages really changed after an insertion — one added page shifts every later page out of step, so they all show as different.

That reflow problem 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 both through OCR PDF 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.

Both files are read in your browser and never leave your device, which matters when the two versions are confidential contracts. The comparison is free and needs no account.