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What is Compare PDF?
Compare PDF lets you compare two PDF documents side-by-side to identify text modifications, added or removed pages, dimensional changes, and visual pixel differences directly in your web browser. Whether checking revised drafts, comparing contract amendments, or quality-checking exported reports, PDFZento calculates structural diffs and pixel changes 100% on-device without uploading your files to any external server.
How to Compare Two PDF Documents Online
Comparing PDF revisions takes just seconds and runs entirely in local browser memory:
1
Upload both PDF documents
Select or drag and drop your original document and revised file into the upload zone above.
2
Select comparison mode
Choose Side-by-Side (Split), Overlay (Blend), or Difference Heatmap to visualize document changes.
3
Configure diff sensitivity
Set sensitivity to Strict (1%), Balanced (5%), or Relaxed (10%), and toggle text, visual, or metadata checks.
4
Review & download diff report
Click Process Compare PDF to view page-by-page changes and download your structured JSON comparison report.
Common Use Cases for PDF Comparison
Document comparison helps teams spot unintended modifications and verify revision accuracy across various workflows:
- Reviewing Revised Contracts & Agreements: Verify that negotiated clauses, dates, numbers, and addenda match agreed terms between contract revisions.
- Checking Financial & Audit Reports: Detect altered figures, updated balance sheets, or modified footnotes between preliminary drafts and final audit copies.
- Quality Checking Design & Print Proofs: Catch subtle typesetting shifts, margin adjustments, image replacements, and font substitutions between export versions.
- Validating Policy & Technical Manuals: Pinpoint added sections, removed procedural steps, and updated diagrams across successive document editions.
Compare PDF vs. PDF Inspector
While both tools examine PDF data, they answer fundamentally different user questions:
| Feature | Compare PDF | PDF Inspector |
|---|---|---|
| Input Documents | Two PDF files (Original vs. Revised) | One PDF file (Single document) |
| Primary Purpose | Identifies changes, additions, deletions, and visual diffs between two versions | Analyzes and reports internal technical specifications and document properties |
| Key Capabilities | Side-by-side split view, pixel diff heatmap, text replacement detection, page status summary | PDF version detection, page geometry (pt), AcroForm fields, annotation count, metadata info dictionary |
| Export Artifact | Structured JSON diff report (pdf-comparison-report.json) | Structured JSON inspection report (pdf-inspection-report.json) |
| Dedicated Tool | Compare PDF | PDF Inspector |
How On-Device PDF Comparison Works
PDFZento performs multi-tier comparison directly inside your browser sandbox:
- Structural Document Analysis: Compares page counts to identify added or removed pages, validates page-by-page dimensions (width and height in points), and checks rotation angles.
- Text Content Diffing: When Compare Text Content is enabled, the engine extracts character sequences page-by-page using PDF.js and flags textual modifications, insertions, or replacements.
- Visual Pixel Difference Engine: When Compare Visual Pixels is enabled, pages are rasterized into high-resolution canvas frames at 1.0 scale. The engine calculates pixel differences across color channels with anti-aliasing noise rejection, identifying exact modified pixel ratios and bounding coordinates.
Frequently Asked Questions
How do I compare two PDF files online?
Upload both the original and revised PDF documents into the upload zone above, select your preferred comparison mode (Side-by-Side, Overlay, or Difference Heatmap) and sensitivity threshold, then click "Process Compare PDF". PDFZento highlights structural changes, text modifications, and pixel differences on-device.
What does the Compare PDF tool check between documents?
The comparison engine performs a multi-layer evaluation: (1) Structural differences such as added or deleted pages, page dimension shifts, and rotation changes; (2) Text content changes extracted page-by-page via PDF.js; and (3) Visual pixel differences with anti-aliasing noise filtering.
Can I compare scanned PDFs?
Yes. Because PDFZento includes visual pixel comparison, it can detect graphical and layout changes across scanned pages even if the scan contains no selectable text layer. For deep text diffing on scans, convert them with our OCR PDF tool first.
What visual comparison modes are available?
You can choose between Side-by-Side (Split view of original and revised pages), Overlay (blended visual alignment), and Difference Heatmap (highlighting altered pixel areas).
How does the sensitivity threshold work?
The sensitivity setting determines the color channel tolerance for pixel differences: Strict (1% threshold) flags even minor anti-aliasing shifts, Balanced (5% threshold, default) catches genuine layout and text changes while ignoring rendering noise, and Relaxed (10% threshold) highlights only significant visual edits.
Can I download a comparison report?
Yes. Once processing completes, you can download a structured pdf-comparison-report.json artifact containing page-by-page change statuses (IDENTICAL, CHANGED, ADDED, REMOVED), modified pixel ratios, and summary metrics.
What is the difference between Compare PDF and PDF Inspector?
Compare PDF analyzes two PDF documents to detect differences, revisions, and page alterations between them. PDF Inspector evaluates a single PDF to report technical structural properties like PDF version, page dimensions, AcroForm fields, annotations, and metadata.
Are my confidential documents uploaded to a server during comparison?
No. All page rendering, text extraction, and pixel difference calculations run 100% on-device inside your browser using WebAssembly and Web Workers. Your documents never leave your device.