Are my confidential contracts uploaded to a server when comparing PDFs?

No. PDFMinty compares your PDF files 100% locally inside your web browser using PDF.js and client-side JavaScript. Neither Version 1 nor Version 2 ever leaves your device.

How does the PDF text comparison algorithm work?

The tool extracts text items with their spatial Y-coordinates on each page to reconstruct reading lines, then runs a Longest Common Subsequence (LCS) matrix algorithm to pinpoint exact line additions, deletions, and similarity percentages per page.

Can I compare two PDF files that have different page counts?

Yes. The comparison engine iterates through the maximum page count across both documents and clearly highlights pages or sections added or removed in the revised version.

What is the difference between Side-by-Side, Unified Diff, and Visual Pages mode?

Side-by-Side displays original and revised text lines in parallel columns; Unified Diff merges changes into a single chronological code-review stream; Visual Pages renders both PDF pages onto HTML5 Canvas elements for layout inspection.

Compare PDF Files Online Free — Private Side-by-Side PDF Diff

Why Local In-Browser PDF Comparison Matters for Sensitive Documents

When reviewing redlined employment agreements, vendor contracts, lease revisions, or regulatory filings, the documents you are comparing are almost always confidential. Uploading two drafts of a non-disclosure agreement (NDA) or merger contract to a remote cloud comparison portal exposes both the original terms and the negotiated edits to third-party servers.

PDFMinty's Compare PDF tool eliminates cloud exposure by executing the entire comparison pipeline locally inside your browser's memory. Using WebAssembly-accelerated PDF parsing and a deterministic Longest Common Subsequence (LCS) diff engine, you can audit every added, deleted, or modified line between two PDF versions without transmitting a single byte over the network.

How Client-Side PDF Diffing Works Under the Hood

Unlike plain-text files, PDF documents do not store paragraphs as simple newline-delimited strings. Instead, a PDF page stream positions individual glyph runs at explicit (X, Y) coordinates across the page canvas. To produce an accurate, human-readable comparison without server-side processing, PDFMinty executes a three-stage local pipeline:

Coordinate-Aware Line Reconstruction: Using pdfjs-dist inside local browser memory, the engine extracts every text item along with its transformation matrix and groups glyphs that share a horizontal baseline (within a 3-point vertical tolerance) into coherent text lines.
Longest Common Subsequence (LCS) Matrix Evaluation: For each corresponding page pair between Document A (Original) and Document B (Modified), the engine computes an LCS dynamic programming matrix over normalized lines. This distinguishes genuine text insertions and deletions from shifted lines.
Dual HTML5 Canvas Rasterization: When you switch to the Visual Pages tab, both versions of the active page are rendered at 1.25x scale onto side-by-side HTML5 Canvas elements so you can inspect typography, signatures, stamps, and table alignment alongside the textual diff.

3 Practical Workflows for Side-by-Side PDF Comparison

Legal & Procurement Contract Redlining: Verify that a counterparty's "clean" final PDF matches the agreed terms from the previous draft and that no clauses, liability caps, or dates were silently altered before you sign with our Sign PDF tool.
Financial & Compliance Audit Verification: Compare quarterly report drafts, invoice revisions, or policy manuals page-by-page, then export a plain-text diff summary report for your audit trail.
Pre-Release Document Sanitization Checks: After running a document through Sanitize PDF, Flatten PDF, or Compress PDF, compare the output against the original file to confirm that all visible body text remained intact.

Comparing Text-Based PDFs vs. Scanned Image PDFs

Digital PDFs exported from Microsoft Word, Google Docs, LaTeX, or accounting software contain embedded font dictionaries and selectable text streams, enabling instant line-by-line diffing and similarity scoring. If you are comparing two scanned paper documents (where each page is a photograph), use the Visual Pages mode to inspect both scans side-by-side, or extract their text first using our OCR PDF or PDF to Markdown tools.

Best Practices After Completing Your Document Review

Once you have verified all changes between Version 1 and Version 2, you can finalize the approved document directly in your browser: apply your signature using Sign PDF, lock interactive form fields permanently with Flatten PDF, strip hidden author and editing history via Edit PDF Metadata, or encrypt the final file with Protect PDF.

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