How to Compare Two Versions of a Contract (and the Changes a Diff Will Not Flag)

You sent a contract out for review. It came back with a new file name, no visible track changes, and a message saying "just a few tweaks, nothing major." Before you sign, you need to know exactly what changed between your version and theirs, not what the cover email claims changed. That is a document comparison, sometimes called a diff or a redline compare, and you can do it properly for free in a few minutes. This guide (published by Adira, which makes contract review and CLM software, so we have a commercial stake in you trusting contracts more, but the steps below work whether or not you ever use our product) walks through three free ways to compare two contract versions, a test you can run afterward to catch what the comparison itself misses, and the honest limits of what a diff tool can and cannot tell you.

Three free ways to compare two contract versions

All three tools below line up Version A and Version B, word by word, and show insertions, deletions, and, in the better tools, moved text. None tells you whether a change is good or bad for you, that judgment stays yours, as the next section shows.

1. Microsoft Word: Compare Documents

If both files are .docx, this is the most reliable free option, and most people already have it.

1. Go to the Review tab, click Compare, then Compare Documents (not "Combine", which merges edits from multiple reviewers rather than diffing two versions).
2. Set Original document to the version you sent, and Revised document to the version that came back. Click OK.
3. Word opens a new document with the differences shown as tracked-change markup, red strikethrough for deletions, underline for insertions.
4. Use the Reviewing Pane (Review tab) to scroll through every change as a list, rather than hunting through the document visually. This is the step most people skip, and it is the one that catches small edits buried in long paragraphs.

Word's compare engine is genuinely good at catching moved text, showing it with a distinct "moved" marker instead of an unrelated delete-plus-insert pair, which weaker tools cannot do.

2. LibreOffice Writer: Compare Document (free, works on .docx too)

LibreOffice is free, works offline, and opens .docx files without converting them first.

1. Open the newer version in LibreOffice Writer, then go to Edit → Track Changes → Compare Document.
2. Select the older version as the file to compare against.
3. LibreOffice shows the differences as tracked changes directly in the document, with a Manage Changes panel listing every edit so you are not relying on catching each one visually.

LibreOffice is a solid free choice, but its move-detection is weaker than Word's: moved paragraphs are more likely to show up as an unrelated deletion in one spot and an unrelated insertion elsewhere, which you have to notice and connect yourself.

3. Google Docs: Compare documents

Useful if both sides already work in Google Docs, or you have converted a .docx into one.

1. Open the base version (the one you sent) in Google Docs, then go to Tools → Compare documents.
2. Under Select comparison document, choose the second file from your Drive (upload it first if it is not there already), and enter a name in Attribute differences to.
3. Click Compare. Google Docs creates a new document showing insertions and deletions as suggested edits; formatting-only differences show up as comments rather than inline marks, so check the comment column too.

The original two files stay untouched, but both pass through Google's servers to run the comparison, which matters if the contract is confidential (more on this below).

4. Weave: mark up both versions without uploading anything

If you want to review a contract without creating an account, converting a file, or sending it to anyone's server, Weave (Adira's free browser tool) lets you open a contract and mark up specific clauses, dates, and numbers as you read, directly on the page. For a comparison, open the sent and returned versions in two tabs or a split screen, and use Weave to highlight and comment on each clause as you check it against the other copy, so your findings are recorded on the document itself rather than in your head. It will not replace an automated line-by-line diff for a long, heavily-edited contract, but it is the option to reach for when the file simply must not leave your machine.

The redline that got through: why running a diff matters

Here is the kind of change a proper diff catches immediately, if you run one.

What you sent: "The Service Provider's aggregate liability under this Agreement shall not exceed the fees paid by the Client in the preceding twelve (12) months."

What came back, unhighlighted, in an email that said "minor edits only": "The Service Provider's aggregate liability under this Agreement shall not exceed the fees paid by the Client in the preceding one (1) month."

That single change turns a twelve-month liability cap into a cap roughly one-twelfth the size, and nothing else in the paragraph looks different. Read at normal reading speed, most people's eyes slide right past "twelve (12)" becoming "one (1)". Run any of the three tools above, and it shows up as a plain, unmissable red-and-blue edit on that exact word. This is the strongest argument for comparing every returned draft rather than skimming it: these tools are genuinely good at catching exactly this kind of change. The real danger is not that diffing fails on obvious edits like this one. It is what diffing does not reliably catch, covered next.

What a diff will not catch, even when you run it

A text diff compares characters and words, not contracts, so it has blind spots that have nothing to do with which tool you use.

• Changed meaning of a defined term. If "Confidential Information" is redefined in Clause 1 to exclude something it used to include, every later use of the phrase is unchanged text. The diff shows zero changes in Clause 9 or Clause 14, even though the term's legal meaning just shifted everywhere it appears.
• Deleted cross-references that were never updated. If Clause 8 is deleted or renumbered, a diff flags that. It will not flag that Clause 15 still says "as set out in Clause 8", now pointing at the wrong provision or nothing at all. Cross-reference checking is not what a diff tool does.
• Moved text that changes legal effect. Content moved from an exhibit into the operative clauses, or from one party's obligations into the other's, can look like an unrelated deletion plus an unrelated insertion in a weak tool, while meaning something completely different in its new location.
• Number and date swaps that read as tiny edits. A "12" becoming a "1", a "shall" becoming a "may", a "not" quietly disappearing. Good tools flag these, as the example above shows, but only if you open the reviewing pane and read every flagged change rather than assuming "looks about the same."
• Anything outside the file you actually compared. If the returned version references a separate exhibit, schedule, or "terms available at [a URL]" that was itself edited, no document-compare tool touches that at all.

The runnable test: check by hand after the diff

Do this every time, it takes under five minutes and catches what the diff tool structurally cannot.

1. Before comparing, write down three or four things that would hurt you most if changed: the liability cap number, the termination notice period, the payment due date, and the definition of any term central to the deal (deliverables, confidential information, territory).
2. Run the diff and read every flagged change in the reviewing pane, not just the ones that visually jump out.
3. Separately, open both original files (not the diff output) and Ctrl+F each of those three or four terms in both documents.
4. Read the full sentence around each hit, side by side, and confirm it says the same thing, not just that the same words appear. A defined term can look unchanged where you searched while its actual definition, elsewhere in the document, has moved.

If anything does not match exactly, raise it before you sign, regardless of what the diff showed.

Red flags in a diff you should never wave through

Normal	Red flag	Why it matters
A paragraph is reworded for clarity; dates and numbers unchanged	A single digit changes inside an otherwise normal-looking rewrite	A one-character edit buried in a rewritten sentence is easiest to skim past
A clause is deleted with a clear strikethrough and a reason given	A clause is deleted with nothing inserted in its place, no explanation	Bare deletions are easy to miss; check every deletion, not just every insertion
A defined term's use is unchanged everywhere the diff shows a mark	The definition itself has changed, but no mark appears anywhere the term is later used	A redefinition silently changes every later use without a single flagged line
Text is moved and the tool shows it as a tracked "move"	Identical text reappears elsewhere as an unrelated delete-and-insert pair	Content has relocated, sometimes into a clause with different legal priority or effect
Clause renumbering is fully reflected in every cross-reference	A clause is added or removed, but a reference elsewhere ("as set out in Clause 14.2") is not updated	The diff flags the renumbering but cannot check whether every reference still points correctly
The liability cap sentence is identical in both versions	The cap is untouched, but a definition elsewhere (what counts as "Fees") has changed	The number controlling your exposure can live in a definition far from the cap clause
The comparison is run on the complete file both sides intend to sign	Only an excerpt is compared, or only visible track-changes are reviewed, ignoring edits made with tracking off	An independent diff catches edits made without track changes; reviewing only the marked-up copy does not

Privacy: comparing on your machine versus uploading to a website

Word's and LibreOffice's compare features run entirely on your computer; the files never leave your machine. Google Docs' comparison runs on Google's servers, since both files have to sit in Drive for the tool to work, which is a reasonable trade for most business contracts but worth knowing if the document carries an NDA or confidentiality obligation restricting where it can be stored. A genuinely in-browser tool, one that does not require an upload to mark up a document, avoids this question entirely.

Many free "compare PDF online" and "diff contract free" websites upload both files to a server you do not control, sometimes with unclear retention terms. Before using one for a confidential contract, open your browser's developer tools (F12), go to the Network tab, and upload a test file. A large upload request firing off to a domain you do not recognise means the tool is sending your document somewhere. For anything genuinely confidential, prefer a local tool or a browser tool that needs no upload.

Keeping your own comparison result also matters legally, not just practically. If a dispute ever arises about which version was actually sent, courts look at the contract document itself, not recollections of what was verbally agreed. Once terms are reduced to a document, Section 91 of the Indian Evidence Act, 1872 (now Section 94 of the Bharatiya Sakshya Adhiniyam, 2023) generally excludes other evidence of what the contract "really" said:

"When the terms of a contract... have been reduced to the form of a document... no evidence shall be given in proof of the terms of such contract... except the document itself, or secondary evidence of its contents in cases in which secondary evidence is admissible..." Source: Section 91, Indian Evidence Act, 1872

The Supreme Court explained why in Bai Hira Devi v Official Assignee of Bombay (AIR 1958 SC 448): Sections 91 and 92 rest on the "best evidence rule", the best evidence of a document's contents is the document itself, not testimony about what someone believed it said. See the full judgment on Indian Kanoon. In practice, the comparison you run before signing is close to your only real chance to catch a change; once both sides sign, arguing that you "thought" the cap was still twelve months, when the signed document says one month, is a hard argument to win. Keep your diff output and the emails carrying both files alongside the signed contract, which also helps if you ever need to prove an electronic record's authenticity under Section 63 of the Bharatiya Sakshya Adhiniyam, 2023 (previously Section 65B of the Evidence Act), the provision governing how electronic records, including emailed drafts, are proved in an Indian court.

Where this fits with the rest of your review

Comparing versions is one step in reviewing a contract, not the whole job. For the broader read-through, see how to read and understand a contract. If you specifically suspect the other side edited the document without track changes on at all, a distinct and common problem, see how to catch untracked changes the other side made to your contract.

FAQ

Can Word's Compare Documents feature work on a PDF? Not directly, it works on .docx files. If you only have PDFs, convert both to .docx first, or use a PDF-specific comparison tool. Spot-check the converted text against the original PDF for a page or two first, conversion occasionally drops or reflows text and produces false differences.

Does a document comparison catch formatting-only changes, like font or spacing? Word and Google Docs generally separate formatting changes from text changes, sometimes as a distinct mark or a comment rather than inline text. Formatting differences are rarely the risk that matters in a contract; focus on text changes first.

What if the returned contract is a scanned image, not a real PDF or Word file? None of these tools can diff an image. Make it searchable text first. See how to turn a scanned contract into searchable text.

Is it safe to use a random free online "contract diff" website for a confidential contract? Be cautious. Many upload your file to a server with no clear data-handling terms. Prefer Word or LibreOffice (fully local) or a genuinely in-browser tool that needs no upload, especially for anything under an NDA.

What is the actual difference between "Compare" and "Combine" in Word? Compare shows the differences between an original and a revised copy of the same document, which is what you want when checking what a counterparty changed. Combine merges edits from multiple reviewers who worked on separate copies of the same original, a different job, useful for your own team's parallel comments, not for checking a returned draft.

Can a diff show "no changes" when something actually did change? Yes. If you accidentally compare two copies of the same file, or the wrong pair of versions, the tool correctly reports no differences between those two files, which tells you nothing about what changed against the version you actually sent. Always double-check the file names and dates before trusting a "no differences" result.

This guide gets you to a reliable way of seeing what changed between two contract versions, and what to double-check by hand afterward. It does not tell you whether a specific change is acceptable for your deal, that depends on your negotiating position and the stakes involved, and is not legal advice. For a change that matters, especially around liability, indemnity, or termination, get a lawyer to look at it before you sign.