How to Compare Two PDFs (Spot Every Change in Seconds)
Two versions of a contract, two drafts of a report — see every difference highlighted side by side. No more eyeballing 50 pages.
You got "v2" of a contract from the other side. They claim "just minor clarifications" — but you remember exactly that's how the last surprise clause got slipped in. Reading 30 pages line-by-line takes an hour and you'll miss things.
PDF compare tools do this in seconds. Here's how it works.
What "compare" actually does
A PDF compare tool:
- Extracts the text from both files, preserving structure (paragraphs, headings, tables).
- Aligns the two extracts — figures out which paragraph in v1 corresponds to which paragraph in v2.
- Diffs at the word level — for each aligned paragraph, shows which words were added, removed, or changed.
- Renders the result as either:
- A side-by-side view (v1 on the left, v2 on the right, with colour highlights), or
- A merged "redline" view (v2 with insertions in green, deletions in red).
It's the same thing Word does with Track Changes — except it works on finished PDFs where neither side has Track Changes turned on.
How to compare two PDFs — step by step
Open the compare tool
Go to pdfty.com/tools/compare. Pro feature — free trial available.
Upload both PDFs
Two drop zones: "Original" (v1) and "New" (v2). Drag a file into each.
Pick view type
Side-by-side (default): v1 on the left, v2 on the right, changes highlighted in colour.
Redline: a single combined document showing v2 with insertions/deletions like a Word Track Changes redline.
Compare
5-15 seconds depending on file size. Larger documents (100+ pages) take longer.
Review and download
Scroll through the highlighted result. Download as a new PDF with all changes marked, or export a plain-text summary of the changes.
What gets highlighted
| Change type | Highlight colour | Example |
|---|---|---|
| Added text | 🟢 Green | A new clause inserted in v2 |
| Removed text | 🔴 Red | A clause from v1 dropped in v2 |
| Changed words (substitution) | 🟡 Yellow | 'shall pay' → 'may pay' |
| Reordered paragraphs | 🔵 Blue | Same content, different position |
| Number changes | 🟠 Orange | A date or amount changed |
| Unchanged text | ⚪ Grey | The 99% of the document that's the same |
Real-world examples
Two versions of a 12-page NDA: Side-by-side view shows that paragraph 7.3 had "30 days" changed to "90 days" (yellow highlight), and a new paragraph 11.2 was inserted (green highlight). Without compare, that change would have been one character in 5000 words of legalese.
A 50-page financial report: Side-by-side reveals that table totals on page 23 changed (because a line item on page 15 was edited). Compare catches both — the upstream edit and its downstream effect.
Two drafts of a research paper: Redline view shows every paragraph reworded between draft 3 and draft 4. Your co-author's "minor edits" turn out to be 40% rewritten — useful information before signing off.
Frequently asked questions
Can it compare scanned PDFs?
Only if both have a text layer. If they're plain scans (no OCR), the compare tool can't see the text. Run both through OCR first, then compare the OCR'd versions.
What if the documents have completely different structures?
Compare still works — it'll just show a lot of "added" and "removed" blocks. Useful for comparing two takes on the same brief, even if neither is a revision of the other.
Will it catch image differences (a chart with new bars)?
No — image-level differences (a chart that looks different) aren't detected as text changes. For that, use a dedicated image diff tool. Our compare focuses on text, where most legal/factual content lives.
Does it work on encrypted PDFs?
Both files need to be unlocked first. Use Unlock on each with their passwords, then compare.
Can I export the changes as a Word document?
Yes — the redline view exports as a Word document with tracked changes, ready to share with collaborators.
Why is this a Pro feature?
Compare is more compute-intensive than most tools — text extraction + alignment + diffing across long documents. We keep it Pro to offset the cost. Free trial available; $9/mo Pro covers unlimited compares.
Is there a file size limit?
Free trial: 10 MB per file. Pro: no limit.
Does it work for PDFs in languages other than English?
Yes — text extraction and word-level diffing work for any language that's written left-to-right or right-to-left, including Cyrillic, Arabic, Chinese (character-by-character diff). Heavy formatting languages (vertical Japanese) may have a slightly noisier output.
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