The term
A redline is a document marked up to show the differences from an earlier version: deleted text struck through, inserted text highlighted or underlined. The name comes from the red pen. In Word the feature is called tracked changes. For PDFs, which carry no change history, the redline has to be computed by comparing two versions.
Producing one from two PDFs
Upload the earlier version and the later version. The engine aligns the text of both documents, computes the differences at character level, and writes the later document back out with each change marked in place: deletions as struck-through red text at the position where they were removed, insertions highlighted in green. The output is a normal PDF that opens in any viewer and can be attached to an email or filed.
Redline versus side by side
A redline is one document with the changes marked in. It is the right format for sending to someone who needs to read the final version and see what moved. A side-by-side view shows both documents with synced scrolling and highlights on each side. It is the right format for a reviewer working through the changes one at a time. Every comparison here produces both, and a change list that links to each highlight.
What a good redline should not contain
- Page numbers and running headers marked as changed on every page.
- Every line of a paragraph marked as changed because one word was inserted.
- Cells of an unchanged table marked because the extractor read them in a different order.
- Formatting changes mixed in with text changes without a way to hide them.
For lawyers
The legal industry page covers the three weekly workflows: redlining the counterparty's returned draft, verifying the execution copy, and checking an amendment against the original.
Try it on your own documents. Upload two versions on the homepage. Documents up to 30 pages need no account, and nothing is stored.
Industry page: PDF comparison for law firms and legal teams