How to compare two versions of a contract
The clause that costs you money is rarely the one that was rewritten. It is the one where a single number moved and everything around it stayed identical.
Why reading them side by side fails
Human proofreading is good at spotting new paragraphs and bad at spotting small substitutions in familiar text. Thirty changed to sixty, "may" changed to "shall", a deleted "not" — these sit inside sentences you have already read three times, and the eye supplies what it expects.
It gets worse with length. By page twelve of a redraft, attention is the limiting factor, and that is exactly where the payment terms tend to live.
First, check the PDF actually contains text
This is the single thing that determines whether any comparison is possible. A PDF exported from a word processor contains real text. A PDF produced by scanning a signed copy contains photographs of pages and no text whatsoever — to a machine there is nothing to compare.
The test is simple: try to select a sentence with your cursor. If nothing highlights, it is a scan, and a diff tool will report two documents that appear identically empty. That needs OCR first, and OCR on a signed scan introduces its own errors, so treat any result from it as a guide to where to look rather than as the answer.
Running the comparison
With two text-bearing files, a comparison gives you a line-by-line diff with the surrounding context — enough to see not just that a clause changed but what it changed from. Start with the page count and the metadata: a redraft that gained a page has gained something, and a document whose author changed has been through hands you may not know about.
Work through the differences in order rather than skimming for the interesting ones. The reason to use a tool is precisely that it does not decide in advance which changes matter.
What a diff will not catch
- Formatting changes that alter meaning — a clause moved under a different heading reads differently while the words stay the same.
- Anything in an annex that was attached separately, or referenced but not included.
- Changes inside embedded images, signature blocks or stamped exhibits.
- Two documents that were rewritten rather than edited, where everything differs and nothing stands out.
Do not upload the contract
This is the part worth being blunt about. A draft under negotiation is confidential, frequently subject to an NDA that names your employer, and uploading it to a comparison site puts a copy on someone else's server under terms nobody read.
Compare it in your own browser, where the files never leave your machine. For anything commercially sensitive, that is not a preference — it is the only version of this task that is safe to do at all.