What is document comparison (a redline)?
Document comparison, often called a redline or blackline, is checking two versions of a document against each other and marking every difference: added, deleted and changed words, formatting changes, and moved or changed pages. Lawyers and reviewers use it to see exactly what the other side changed in a draft before agreeing to it.
Also called: Redline, Redlining, Blackline, Compare documents
Updated October 15, 2026 · By the PDFZyro team
Where the word redline comes from
Before software, reviewers marked changes by hand in red ink. Today a redline usually shows insertions in colour, often underlined, and deletions struck through, so you can read the new version and see what changed at the same time. Some firms and countries call the same thing a blackline.
Comparison vs tracked changes
Tracked changes in a word processor record edits as they're made, but only while tracking is switched on, and changes can be accepted or hidden before the file is sent. A comparison doesn't depend on how the document was edited: it looks at the two finished versions and finds every difference, including changes made with tracking turned off. That's why it's good practice to compare the version you receive against the one you sent, even when changes are tracked.
Comparing PDFs
PDFs are often the final form of a contract, report or drawing, so comparing them is common. A good comparison finds text changes in context, formatting changes that can make a clause look less important, pages that were added, removed or moved, and changed pictures. Scanned PDFs have no text to compare, so they need OCR first, or a visual comparison of the page images.
When a contract comes back from the other side, compare it with the version you sent before you sign. It takes minutes and catches changes that are easy to miss when reading, such as a different number or date, or a single word like may instead of must. A report listing every change with its page is also a useful record to share with colleagues or a client.
How to redline a PDF
There are two ways to redline a PDF. To see what changed between two versions, run a comparison: choose the original and the revised PDF, and the tool marks every added, deleted and changed passage. To propose changes yourself, mark up the PDF by hand with comments: strike through the words to delete, underline or add a note with the new wording, and send it back. Either way, keep the clean versions, so the redline document is only a record of the changes.
How to redline two PDFs in PDFZyro
- Open Compare and choose the older and the newer version: two different files, or two versions of the same document.
- The comparison runs in the background, for documents of up to 500 pages.
- Step through each difference with the two versions side by side, then download a report of every change to share, with removed text marked in red and added text in green.
- To mark up changes by hand instead, open the PDF and click Comment: use Strikethrough, Underline and sticky notes, then save the marked-up copy as a new version.
Tools and guides
- Guide: How to compare two PDF filesHow to compare two PDF files: the ways to do it, step-by-step instructions, what a good comparison finds, and tips for comparing contracts and scans.
- Compare PDFSee every difference between two versions of a document.
- OCR PDFMake scanned PDFs searchable and editable.
Related terms
- OCR (optical character recognition)Software that reads the text in scans and photos so it can be searched and copied.
- RedactionPermanently removing sensitive text or images from a document before it is shared.
- Electronic signature (e-signature)Any electronic sign that shows a person agrees to a document, from a typed name to a drawn signature.