How to Compare Two PDF Documents for Free (Word-by-Word)

When You Need to Compare PDFs

The most common scenarios: a lawyer reviewing two versions of a contract to spot clause changes before signing. A researcher comparing a pre-review paper with the revised version. A procurement team checking whether vendor terms changed between drafts. An HR department verifying that a policy update only changed the intended sections.

In all of these cases, you need to see exactly what changed — not just that something changed. Word-level highlighting tells you whether "shall" became "may" in a contract clause, which matters. Line-level comparison that shows an entire paragraph as "changed" doesn't.

The Free Method: Step by Step

ToolLance's PDF Diff tool uses the open-source PDF.js library for text extraction and a Longest Common Subsequence (LCS) algorithm for word-level diffing. Everything runs in your browser — no files uploaded, no account, no cost.

Step 1: Go to toollance.com/tools/pdf-diff.

Step 2: Drag your original PDF into the left drop zone and your modified PDF into the right drop zone. Or click each zone to browse files.

Step 3: Click "Compare PDFs." The tool extracts text from both files (this takes a few seconds for large documents) and runs the comparison.

Step 4: Review the results. At the top, you'll see:

• Similarity % — how alike the two documents are overall
• Unchanged lines — count of identical lines
• Changed lines — lines that differ between versions
• Added lines — lines in the new version not in the original
• Removed lines — lines in the original not in the new version

Below the summary, you can toggle between Side-by-Side view(both PDFs in parallel columns) and Unified view (all changes in a single column with old/new labels).

Understanding the Highlighting

Within changed lines, individual words are highlighted at the word level:

• Green highlight — words added in the new version
• Red highlight with strikethrough — words removed from the original
• Yellow row background — lines that changed (even if only one word changed)

This means if a contract clause changes one word from "shall not" to "may not", only those two words are highlighted — not the entire clause. This is more useful than Adobe Acrobat's default view, which often highlights entire paragraphs when the text reflows.

The Limitation You Need to Know

This tool works with text-based PDFs — documents created in Word, Google Docs, or exported from web pages. It doesn't work with scanned PDFs.

Scanned PDFs are images of text, not actual text. PDF.js can only extract text that was encoded as text when the PDF was created. For scanned documents, you'd need OCR (Optical Character Recognition) software to extract the text first, then compare the text output.

If you're not sure whether your PDF is text-based: try selecting and copying text from it in a PDF viewer. If you can copy text, the diff tool will work. If the text appears as an image you can't select, it's a scanned PDF.

Why "No Upload" Matters for This Use Case

PDF comparison is heavily used for confidential documents. Contracts, legal filings, medical records, financial statements, HR policies. Uploading these to a third-party cloud service creates a data exposure risk — you don't know where the data goes, how long it's stored, or who can access it.

ToolLance's PDF Diff tool processes files locally. When you click "Compare PDFs", the JavaScript on the page reads both files from your device's memory and runs the comparison there. No network request is made with your file data. You can verify this in your browser's network tab (DevTools → Network) — you'll see no outgoing requests with PDF data.

If you need to convert an image to PDF before comparing (e.g. a photo of a signed document), the Image to PDF converter works the same way — paste, convert, download, all locally.