Affiliate disclosure: This guide contains affiliate links. If you purchase through them, we may earn a commission at no extra cost to you.
How to Blackline a PDF: Legal Blackline Comparison Explained (2026)
If you work in legal, you’ve heard the term blackline — but what does it actually mean, and how do you produce one from a PDF? This guide explains blacklining, how it differs from redlining, and the tools that generate proper legal blacklines from PDF documents.
Quick answer: A blackline is a comparison document showing every change between two versions of a contract — the term comes from legal practice (Microsoft Word even names its compare feature “legal blackline”). To blackline a PDF, use Adobe Acrobat Pro’s Compare Files tool (generates a marked-up report) or Diffchecker’s Legal plan (purpose-built redline/blackline output with OCR). Free tools like DiffPDF highlight differences but don’t produce a formal blackline document.
Blackline vs Redline: What’s the Difference?
The terms are often used interchangeably, but strictly speaking:
- Redlining = marking up a document as you edit it (the red pen, or Word’s Track Changes).
- Blacklining = comparing two finished versions to reveal every difference — historically done by overlaying the original and the marked-up copy.
In practice, when a lawyer asks for “a blackline,” they want a comparison document: the new version with all changes marked. Word’s Review > Compare > “Compare two versions of a document (legal blackline)” does this for .docx files. For PDFs, you need a PDF comparison tool.
Method 1: Adobe Acrobat Pro (The Standard)
Acrobat Pro’s Compare Files tool is the closest thing to an industry-standard PDF blackline: Tools > Compare Files, select old and new PDFs, and it outputs a summary report plus a marked-up document with filters for text, formatting, annotations, and images.
Try Adobe Acrobat Pro Free Step-by-step tutorial →
Method 2: Diffchecker Legal (Purpose-Built)
Diffchecker offers a dedicated Legal plan that produces polished, customizable Word and PDF redlines/blacklines — comparing PDF to PDF, PDF to Word, or Word to Word, with tracked changes down to the word and character, plus OCR for scanned documents. If blacklines are your daily work rather than an occasional task, a purpose-built tool pays off.
Method 3: Word’s Legal Blackline (for .docx)
If your documents are in Word rather than PDF: Review tab > Compare > Compare two versions of a document (legal blackline). Select the original and revised documents, choose comparison options (comments, formatting, text), and Word generates a third document with changes marked. Free if you already have Microsoft 365 — but it doesn’t work on PDFs.
Can Free Tools Make a Blackline?
Honestly: not really. The free DiffPDF v2.1.3 highlights differences on screen, but it doesn’t export a marked-up blackline document suitable for sending to a counterparty. For informal internal checks it’s fine; for anything client-facing, use Acrobat or Diffchecker. See our legal professional’s guide for the full workflow.
Frequently Asked Questions
What is a blackline in legal documents?
A blackline is a comparison showing every change between two versions of a document — typically the revised version with insertions and deletions marked. It lets reviewers see exactly what changed without reading both versions fully.
How do I blackline two PDFs?
Use Adobe Acrobat Pro’s Compare Files tool (Tools > Compare Files) or Diffchecker’s Legal plan. Both generate marked-up comparison documents from two PDFs. Free highlight-only tools like DiffPDF don’t produce formal blacklines.
Is blacklining the same as redlining?
Closely related but not identical: redlining is marking edits as you make them; blacklining is comparing two finished versions to reveal all differences. In casual legal speech the terms are often interchanged.
Last updated: October 2026. This guide is for informational purposes and is not legal advice. diffpdf.org is an independent resource center, not affiliated with Adobe or Qtrac Ltd.