Compare two PDF files and see exactly what changed

Compare two PDFs in five steps

1. Open Compare PDF in any up-to-date browser, on a phone or a computer. Nothing to install, no sign-up.
2. Add the older file under Original and the newer one under Changed version. You can also pick both at once, and the older file goes in as the original. "Older" means the date the file was saved on your device. For a download, that is usually when you downloaded it, so glance at the names, and press the swap button if they went in the wrong way round.
3. Wait a moment. The comparison starts by itself once both files are in, and a bar shows how many pages have been read.
4. Read the summary. It says how many changes there are, split into added, removed and changed.
5. Check each change. Use the up and down arrows in the toolbar, or the N and P keys, to go from one change to the next, or switch to List to see them all as cards.

Every button is explained in the tool's step-by-step guide. This article goes further: how the changes are found, how to read them without missing one, and what to do with scans and long files.

How the tool finds the changes, in plain words

A PDF saved from a document keeps its text, and where each word sits on the page. The tool reads the words of every page of both files and lays each file out as one long line of words, from the first page to the last. Then it looks for the longest runs of words that are the same in both, in the same order. Whatever is left between those runs is a change.

Say the original says: The rent is Rs. 15,000 per month, payable by the 5th of each month. The new version says: The rent is Rs. 16,500 per month, payable by the 1st of each month. Everything matches except two spots, so you get two changes, both marked as changed: 15,000 became 16,500, and 5th became 1st. A number like 15,000 counts as one word, so you see the whole old figure and the whole new one.

Three things follow from comparing one long line of words:

• Pages don't matter. If a new sentence on page 1 pushes all the text after it down, the words after it are still the same words in the same order. In our test with 21-page files, that gave exactly 1 change: the added sentence.
• Punctuation counts. A full stop, a comma or a bracket is a word of its own, so a moved comma is found. In a contract, a comma can change what a sentence means.
• Layout doesn't count. The same text saved again with other margins, another paper size or a new font size gives no changes. When we laid out 21 pages again on Letter paper, so they became 20 pages, the result was "No differences in the text".

Repeated headers and footers, like "Confidential" or "Page 3 of 20", are taken out and compared on their own, so page numbers don't show up as hundreds of changes. A header that really changed, say from "Version 1" to "Version 2", is reported once. The details are in how the changes are found.

How to read the result without missing anything

Start with the summary

The summary answers the big question first: how many changes, or No differences in the text. That second answer means both files have the same words in the same order. If you picked the very same file twice, it adds "They are also exactly the same file, byte for byte." Read any notes under it. One warns when fewer than 35% of the words match, which usually means you picked two different documents. Another says how many pages have no text at all, because changes on those pages can't be listed.

Know the three marks

In the Pages view, the original is on the left and the changed version on the right. Each kind of change has its own colour and its own shape, so you can tell them apart even if colours are hard for you to see:

• Added: green, underlined. Only in the changed version.
• Removed: red, struck through. Only in the original.
• Changed: yellow, with a dashed border. The old words on the left became the new words on the right.

Where words were added or removed, the other side gets a small marker at the spot where they would be.

Step through every change, in order

Don't scroll and look for colour. Press the down arrow (or N) and let the tool take you from the first change to the last. The current one gets a blue outline, both sides scroll to it, and a line above the pages says what it is in words, such as: Change 1 of 5: changed on page 1. Counting along this way is the surest way to see every one.

Use the List for an overview

List shows each change as a card with the words before and after, and a few words around them. If the text moved to another page, the card shows both page numbers, like Page 7 → 8. Tap a card to see it on the page. On a phone, this is often the quickest way to check. A very long list shows the first 1,000 changes.

Keep a record

When there are changes, Copy list and Save report give you every change as plain text: its kind, its page in each file, and the words before and after, along with the options you used. The report always has every change. Paste it into an email, or keep the text file with your papers.

What it compares, and what it doesn't

The main comparison is about text: words, numbers and punctuation, in order. It does not compare formatting or pictures. For those there is a second check, Compare the look, which compares pictures of the pages and marks the areas that look different.

So "No differences in the text" is a strong answer about the words, and only about the words. If a stamp, a signature or the look of the page matters, run Compare the look as well. The full list is in what it can't see.

Comparing scanned PDFs

A scan is a picture of a page, with no words inside. When one or both files are scans, the summary says so, "These PDFs have no text to compare", and offers Compare the look. That draws each page of both files as a picture, lines the two up, because a scan can sit a little off, and marks the areas that differ. Soft letter edges and a slight shift don't count. A page that is only in one version is reported too.

In our tests, two 3-page scans with one word changed on page 2 showed a difference on page 2 only, in 0.5 to 0.7 seconds. A 42-page, 50 MB scan against a copy of itself took 16 to 20 seconds and found that the pages look the same.

It works best when both scans were made the same way: the same scanner or app, the same settings, the same paper size. A scan against a PDF saved from a document never lines up exactly, so most of each page gets marked. Two phone photos taken from slightly different angles may not line up well either. And it can tell you where pages look different, not which words changed.

Some scans carry hidden text from text recognition (OCR). The word comparison then uses it, but a letter misread in one copy shows up as a change, so check those against the page.

Only one version is a scan? Ask for the version saved straight from the document; that is the best fix. Otherwise, a text recognition tool such as Google Docs (opening the scan from Google Drive) can turn it into text you save as a PDF. Because the comparison ignores layout, you can then compare that against the other version word by word. Expect a few false changes where letters were misread, and keep in mind that this route uploads the scan to Google.

Long documents: what to expect

Long files are where a tool beats reading by eye, and where you want to know it won't miss anything. We tested with PDFs in which we knew every change:

• 148 pages each, 6 known changes. All 6 found, in 1.8 to 2.8 seconds.
• 298 pages each, the same 6 changes. All 6 found, in 3.7 to 5.8 seconds, 4.6 seconds in the middle run.
• A 117-page report (5.3 MB) against a compressed copy (1.8 MB). No differences, in 3.7 to 5.1 seconds.

These ran in Chromium 141, the open-source browser Chrome is built on, on a Linux computer with 4 processor cores and 16 GB of memory, five runs each on a freshly opened page. Most of the time goes into reading the text of every page, which depends on how the PDF was made as much as on its length: the 117-page report has fewer words than the 148-page document, yet took longer. Finding the changes once the words are read took 0.03 to 0.13 seconds. A phone will usually take longer. All the results are in the tool's real results.

Tips for big files:

• Let it read. The bar shows the pages read. If you picked the wrong file, Cancel stops it, and your files stay ready.
• On a phone, close other apps and tabs to give it memory. Pages are drawn only as you reach them, which keeps memory use low.
• Work from the List and the report. On a long pair, the report is easier to go through than scrolling hundreds of pages, and it has every change even when the list shows only the first 1,000.

Why it matters that nothing is uploaded

Think about what people compare: contracts, rent agreements with home addresses, offer letters with salaries, bids before a deadline, a thesis before it is submitted. Most online PDF tools upload your files to their servers and work on them there. Your documents then sit, at least for a while, on a computer you don't control.

Compare PDF reads, compares and draws both files inside your browser. There is no upload step and no account. You can test it yourself: once the page has loaded, switch off Wi-Fi and mobile data and compare two files. It still works for most PDFs, because the tool is already on your device. A few files need an extra part of the PDF reader the first time, such as fonts for Chinese or Japanese text. In our tests, none of the requests the page made after loading carried a file. Our privacy policy has the details.

For contracts in particular, our guide to checking a revised contract before you sign has a full checklist.

Other ways to compare, and when they are better

• Microsoft Word's own Compare feature. If you have both versions as Word files, Word can compare them and show the changes as tracked edits you accept or reject one by one, formatting included. For a draft you are still editing, that is the better tool.
• Tracked changes plus a check. When someone sends a Word file with tracked changes, ask for the final PDF as well, and compare that PDF with the version you agreed. That shows anything that changed without being tracked.
• Desktop PDF software. Full desktop PDF programs, such as the paid version of Adobe Acrobat, have compare features of their own.
• Reading side by side. You can open each file in our PDF viewer in two tabs. It is fine for a one-page letter, but on anything longer it is slow and easy to miss a changed figure.

Have a Word file and a PDF? Only PDFs can be compared here, so save the Word file as a PDF first. Compare PDF is the right choice when what you have are final PDFs, when you are on a phone, when you can't install anything, or when the files are private.

When the result looks wrong

Hundreds of changes for a small edit

• Check that Ignore extra spaces and line breaks is on. With it off, every line that breaks in a new place counts. In our test, 21 pages laid out again gave 830 changes with it off and none with it on.
• If one file came from a different program, its text may be stored in another order, for example in two columns. The List view helps you pick out the real changes.
• A PDF exported from Word with tracked changes showing has the struck-out words and comment balloons printed on its pages, so they count as text. Ask for the clean version.
• When a clause or a footnote is added, the numbers of the ones after it can move up by one, and each renumbered "7" becoming "8" shows as a small change. They are real, but quick to check.

A change you know about is missing

It may be formatting, a picture, a stamp, text in a form box or a comment, which the word comparison doesn't see: try Compare the look. It may be on a page without text, which the summary mentions. Or Ignore capitals may be hiding it.

Everything is the wrong way round

If new text shows as removed, the newer file went in as the original. Press the swap button and it compares again the other way.

Strange letters in the list

Some PDFs, often ones typed with older Hindi fonts, store their letters in a code of their own. Use the Pages view, which shows the real pages, or Compare the look.