How to Compare Two PDFs and Find Differences

You send someone a contract. They send it back with "minor changes." You open both files side by side, squinting at the screen, trying to figure out what they changed. Sound familiar?

Comparing PDFs manually is painful. Words shift slightly, sentences get reworded, and tracking changes by eye is a recipe for missing something important. The good news: there are free tools that do this for you.

When You Actually Need to Compare PDFs

Let me think through the most common situations where PDF comparison saves your bacon:

• Contracts and legal documents — did the other party change the terms?
• Revisions and edits — tracking what your editor or colleague changed
• Invoice verification — making sure you got charged the right amount
• Form submissions — comparing before/after states
• Compliance — proving what changed between document versions

Basically, any situation where you receive a modified PDF and need to know exactly what changed.

Method 1: Browser-Based Comparison (Easiest)

The simplest approach uses online tools that handle comparison in your browser. You upload both PDFs, and the tool highlights differences.

PeacefulPDF's compare tool does exactly this. Upload your original and revised PDF, and it shows you exactly what changed — text modifications, additions, deletions. Everything stays on your device; nothing gets uploaded to any server.

This is genuinely useful for most comparison needs. It's faster than manual checking and doesn't require installing anything.

Method 2: Using Microsoft Word

Here's a trick most people don't know: Microsoft Word can compare PDFs. Here's how:

1. Open Microsoft Word
2. Go to Review → Compare
3. Select your original PDF as the "Original document"
4. Select the revised PDF as the "Revised document"
5. Word will open a combined view showing all changes

Word treats the PDFs as if they were Word documents and shows track changes-style markup. This is surprisingly powerful if you already have Word.

The downside: Word's PDF handling isn't perfect. Complex layouts might not compare perfectly. But for text-heavy documents, it works well.

Method 3: Adobe Acrobat Pro

If you have Adobe Acrobat Pro, it has built-in comparison tools:

1. Open your PDF in Acrobat Pro
2. Go to View → Compare Files
3. Select your original and revised files
4. Acrobat shows a summary panel with all differences

It's genuinely good — Adobe invented the PDF format, so their comparison is thorough. But at $20-30/month, it's expensive just for comparison. Only worth it if you're already paying for Acrobat.

Method 4: Command-Line Tools (For Automation)

If you need to compare PDFs as part of a workflow or process lots of documents, command-line tools exist:

diff-pdf

This tool creates a visual PDF showing differences:

diff-pdf --output=diff.pdf original.pdf revised.pdf

The output PDF highlights differences visually. You can also use --view to see the differences immediately.

cmp and pdftotext

For a text-focused comparison, convert both PDFs to text and use diff:

pdftotext original.pdf original.txt pdftotext revised.pdf revised.txt diff original.txt revised.txt

This shows you exactly which text changed, line by line. Great for tracking document edits programmatically.

What About Scanned PDFs?

Here's the catch: comparing scanned PDFs is much harder. Scanned documents are essentially images, not text. Before comparing scanned PDFs, you need OCR to extract the text.

If you have scanned documents:

1. Run OCR on both PDFs to make them searchable
2. Then use the comparison tools above

PeacefulPDF's OCR tool can convert scanned PDFs to searchable text first. It's an extra step but necessary for comparing scanned documents.

Visual vs. Text Comparison

Different tools approach comparison differently:

Visual Comparison

Looks at the actual PDF rendering — fonts, spacing, layout changes. Good for catching formatting changes and things that might not show up in text diffs. Tools like diff-pdf do this.

Text Comparison

Extracts the text content and compares what's written. Catches word changes, additions, deletions. Doesn't care about formatting. Word and most online tools do this.

Both have their place. For contracts and text documents, text comparison usually catches what matters. For designed documents, visual comparison catches layout shifts.

Limitations to Know About

PDF comparison isn't perfect. Here's what can trip you up:

• Reformatting: If someone rewrote a paragraph entirely, comparison tools might show it as "deleted old text, added new text" rather than a change
• Images: Many tools don't compare embedded images
• Links and metadata: Changes to links or document properties often go unnoticed
• Complex layouts: Multi-column documents can confuse comparison tools

For critical documents, I always recommend at least spot-checking manually after using a comparison tool.

My Recommendation

Here's how I'd approach PDF comparison:

• Quick checks: Use PeacefulPDF's compare tool — fast, free, private
• Thorough review: Use Word's comparison if you have it
• Automation: Use diff-pdf or text extraction for workflows

Stop trying to spot differences by eye. It's inefficient and error-prone. Let the tools do the work.