AI Document Comparison
Upload two PDFs and AI identifies all differences, additions, and changes.
Upload both PDFs
Drop in the two PDF versions you want to compare — original and revised.
AI analyses the differences
Our AI compares text, structure, and content across both documents and identifies every change.
Review the diff report
A clear difference report highlights what was added, removed, or changed — with a similarity score.
Why SynthPDF?
AI-powered semantic diff
Detects rephrased sentences and moved paragraphs — not just character-by-character changes that miss meaning.
Similarity score
Get a 0–100% similarity score at a glance. 95%+ means near-identical; below 50% means major rewrite or wrong file uploaded.
Tracks additions, deletions, edits
Every added sentence, deleted clause, and modified number is flagged separately — colour-coded in the diff report.
Works on scanned PDFs
Scanned documents are OCR'd automatically before comparison. Accuracy depends on scan quality (150 DPI+ recommended).
Built for contracts and legal docs
Compare redlined contracts, regulatory updates, and policy revisions — identify exactly what changed between versions.
Free, no account needed
Up to 10 comparisons per month on the free plan. Pro and above have higher limits.
When to Use PDF Comparison
Comparing two versions of a PDF manually is tedious and error-prone — especially for long documents. AI comparison is useful whenever you need to:
- Contract redlining — identify what changed between a draft and a revised version
- Regulatory document review — compare last year's policy against the updated version
- Academic paper revision — see what an editor changed after peer review
- Legal discovery — verify that a document produced in discovery matches the expected version
- Quality control — confirm that a final PDF matches the approved draft with no unintended changes
What the AI Detects
The comparison analyses text at the semantic level, not just character-by-character:
- Added text — new sentences, paragraphs, clauses, or sections
- Deleted text — content present in the original but removed in the revision
- Modified text — sentences that were rephrased, numbers changed, or terms substituted
- Structural changes — section reordering, heading renaming, paragraph merging
Layout-only changes (font size, margins, spacing) that don't affect content are reported separately and don't reduce the similarity score.
Understanding the Similarity Score
The similarity score (0–100%) reflects the overlap of text content between both documents:
- 95–100% — near-identical; only minor edits (typo fixes, formatting changes)
- 80–95% — minor revision; a few clauses changed, small additions
- 50–80% — significant revision; sections rewritten or added
- Below 50% — major rewrite or the wrong documents were uploaded
How It Works on Scanned PDFs
Scanned PDFs don't contain selectable text — they're images. The comparison tool runs OCR on both documents first to extract the text, then performs the diff. Comparison accuracy depends on scan quality. For best results, ensure scans are at least 150 DPI and the text is not handwritten.
Preparing Documents for Best Results
- Make sure both PDFs are text-based (not image-only) for highest accuracy
- If comparing Word documents, convert both to PDF first using Word to PDF
- Remove passwords before comparing using Unlock PDF
- For very long documents (100+ pages), focus on specific page ranges for faster, more precise results
Frequently Asked Questions
Text additions and deletions, rephrased sentences, moved paragraphs, and structural differences. Layout-only changes (like font size) are noted separately.
Yes — scanned PDFs are OCR'd first, then compared. Accuracy depends on scan quality.
100% means the documents are textually identical. Lower scores indicate more differences — useful for versioning or plagiarism checks.
Yes — convert your Word file to PDF first using our Word to PDF tool, then compare.
Yes — up to 10 comparisons per month on the free plan. Pro and above have higher limits.