Comparing two versions of a document by eye is slow and error-prone — a changed number here, a reworded clause there, a paragraph quietly deleted. AI can do the reading for you, but there's an important distinction to get right first: do you want a character-level diff that highlights every edit, or a semantic comparison that answers "what actually changed and does it matter"? This guide covers how to compare two PDFs with AI for the second kind — the questions a plain diff can't answer — with a citation on every difference so you can verify it.
Two kinds of "compare"
- Structural diff — a tool that lines up two documents and marks every textual change: insertions, deletions, edits. Great for catching that something changed. Sidenote's document diff tool does this kind of side-by-side comparison.
- Semantic comparison — asking an AI what the meaningful differences are: "which clause changed and how," "what's in v3 that wasn't in v2," "do these two policies contradict each other." This is where AI chat across both documents shines, because you're asking a question, not scanning a redline.
Both are useful. If you need to see every edit, use a diff. If you need to understand the differences and act on them, read on.
How to compare two PDFs with AI
1. Open both versions
Open the two PDFs — the previous and current contract, the old and new spec, two vendors' terms — in your browser. With Sidenote there's no upload or conversion; it reads the files where they already are.
2. Group them into one collection
Put both documents into a single Collection so they can be queried together as one set. (Collections are part of Pro, which has a 7-day trial, no card.)
3. Ask the comparison question
Ask directly: "What changed between these two versions?" "Which clause has the shorter notice period?" "What's in the new one that wasn't in the old?" The answer draws on both documents at once.
4. Verify each difference at its source
This is the step a summary alone can't be trusted without. Every claim in the answer carries a citation naming which document and which passage it came from — click it and that PDF scrolls to the exact sentence and highlights it. So "clause 7 changed from 30 days to 14 days" comes with a one-click path to clause 7 in both versions.
Diff vs AI comparison: which to use
| You want to… | Use | Example |
|---|---|---|
| See every textual edit | A structural diff | Document diff |
| Understand what meaningfully changed | AI comparison across both | Sidenote Collections |
| Answer "which is better on X" | AI comparison, cited | Sidenote Collections |
| Verify a specific change | Either — but cited AI links you to it | Sidenote citations |
For more on questioning several documents at once, see how to chat with multiple PDFs.
Frequently asked questions
Can AI tell me what changed between two contract versions?
Yes. Group both versions into one set and ask what changed, which clauses differ, or what was added or removed. A tool like Sidenote answers across both documents and cites the exact passage in each version, so you can jump straight to the clause that changed and confirm it. For a full character-level redline, pair it with a structural diff.
Do I need to upload both PDFs somewhere to compare them?
Not with an in-browser reader. Sidenote reads both PDFs where they already live, read-only, and groups them into a Collection without copying them into a separate system — so there's no upload step and your existing access stays intact.
How do I trust an AI comparison of two documents?
Insist on citations. Sidenote ties every stated difference to the exact passage in each document and drops claims it can't ground in the text, so you verify each change in one click rather than trusting a blended summary.