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PDF Diff Checker

Upload two PDF files to compare their text content. Highlights added lines, removed lines, and unchanged lines — all in your browser, nothing sent to any server.

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Original PDF (A)

Click or drag & drop a .pdf file

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Modified PDF (B)

Click or drag & drop a .pdf file

About PDF Diff Checker — PDF Diff Checker Online

The PDF Diff Checker extracts the full text from two PDF files and runs a line-by-line comparison, showing you exactly which lines were added, removed, or unchanged between versions. It uses Mozilla's PDF.js library to parse PDFs entirely in your browser — your files never leave your device. The result is displayed in the same familiar color-coded format as a code diff, making it easy to spot contract clause changes, report edits, or documentation updates at a glance.

Common uses include comparing two versions of a legal contract to find inserted or deleted clauses before signing, reviewing tracked changes between a first draft and a final approved report, auditing regulatory document revisions when your organization must maintain a change log, verifying that a PDF export from a system matches an expected template, and cross-checking a client's redlined version against your original. Because the diff is text-based, it works best with PDFs that contain selectable text — scanned image-only PDFs will not produce useful results without an OCR step first.

How to Use PDF Diff Checker

  1. Click the Original PDF (A) upload zone (or drag and drop) to load the first PDF file.
  2. Do the same for the Modified PDF (B) zone to load the second PDF.
  3. Click Compare PDFs. PDF.js will extract text from all pages of both files, then run the diff algorithm on the extracted lines.
  4. Review the color-coded diff — green lines with a + prefix were added in the modified version, red strikethrough lines with a - prefix were removed from the original.
  5. Switch between Unified View (all changes in one scrollable list) and Side-by-Side View (original on the left, modified on the right) using the toggle buttons above the results.

Features and Options

The PDF Diff Checker provides the same diff views used by developer tools, applied to PDF text content for a familiar and readable comparison experience.

  • Unified View: Shows all lines from both documents in a single vertical list. Added lines have a green background with a + prefix. Removed lines have a red background with a - prefix and strikethrough text. Unchanged lines appear without any highlight. This mirrors the output of Git's diff command.
  • Side-by-Side View: Places the original document's lines in the left column and the modified document's lines in the right column. Changed line pairs are highlighted on both sides, and empty placeholder cells (∅) appear where lines exist only in one version. This view is easier to read for longer documents.
  • Summary stats bar: Displays the total count of added lines, removed lines, and unchanged lines at the top of the results so you can gauge the extent of changes before reading the full diff.
  • Multi-page extraction: PDF.js processes all pages of each document and concatenates the text into a single stream before comparing. Page breaks do not interrupt the comparison — the entire document is diffed as a continuous text.
  • LCS diff algorithm: Uses the same Longest Common Subsequence algorithm as professional code diff tools to find the minimal set of changes, minimising false positives and producing the most readable output.

Tips for Getting the Best Results

PDF text comparison has specific characteristics worth understanding before interpreting the results.

  • Test with text-selectable PDFs: Before running the comparison, open each PDF in your browser or reader and try to select and copy a sentence. If you can select text, the PDF has an embedded text layer and this tool will work well. If you can't select text, the PDF is image-based and will produce no meaningful output without OCR first.
  • Expect more "changed" lines than expected for reflowed paragraphs: PDF text is extracted line by line based on the visual layout. If a single word was added early in a paragraph, the entire paragraph may reflow and appear as many changed lines. This is a characteristic of PDF text extraction, not a bug. For cleaner paragraph-level diffs of reflowing documents, use the Word Diff Checker instead.
  • Use Side-by-Side View for long documents: When comparing long contracts or reports, the Side-by-Side View makes it easier to read both versions of a changed passage simultaneously without losing context.
  • Split large PDFs if needed: For very long documents (100+ pages), consider using the PDF Splitter tool to isolate the section you need to compare, then upload just those sections here. This speeds up extraction and makes the diff output more focused.
  • Check headers and footers: PDF.js extracts all visible text including headers, footers, and page numbers. If these differ between the two files (e.g., different dates in footers), they will appear as changed lines. This is expected and can usually be ignored when reviewing the substantive body content.

Why Use a PDF Diff Checker Online

A browser-based PDF diff checker requires no installation, no Adobe Acrobat subscription, and no command-line tools. You do not need specialist software or technical knowledge — just upload two PDF files and click Compare. Because PDF.js processes files entirely in your browser, your documents are never uploaded to any server, which is critical for legal, medical, financial, and regulatory documents that must remain confidential and cannot pass through third-party systems.

Legal professionals benefit by comparing redlined contract versions without exposing confidential client documents to cloud services. Compliance teams use it to audit regulatory document updates. Technical writers compare documentation releases. Procurement staff check that a supplier's updated terms match the agreed version. Any organization that manages versioned PDF documents as part of its workflow will find a visual text diff far faster and more reliable than manual side-by-side reading.

Frequently Asked Questions about PDF Diff Checker

No. The tool uses Mozilla PDF.js, an open-source library that runs entirely in your browser. PDF files are loaded from your local file system into browser memory, parsed in JavaScript, and the extracted text is compared locally. Nothing is ever sent to any server, making this safe for confidential documents such as contracts, medical records, financial statements, or any document that must not leave your device.

No. This tool extracts embedded text from PDFs. Scanned PDFs are image-based — the pages are photographs of documents with no underlying text layer, so PDF.js cannot extract readable text from them. To compare scanned PDFs, first run them through an OCR (Optical Character Recognition) tool such as Adobe Acrobat, Google Drive, or a free OCR service to create text-based PDFs, then upload the OCR output here.

PDF text is extracted based on visual line positions on the page. If a word is inserted early in a paragraph, the paragraph may reflow and every line in it will have different content, appearing as many changed lines in the diff even though the actual change was small. This is an inherent characteristic of how PDFs store text. For documents with flowing paragraphs, the Word Diff Checker (which compares whole paragraphs) may produce a cleaner, more readable result.

No. Password-protected PDFs cannot be parsed without the password. To compare a protected PDF, open it in your PDF reader (Adobe Acrobat, Preview on macOS, or your browser), enter the password, then export or print it to a new unprotected PDF file. Upload that unlocked copy here. Never share or upload PDFs in a way that would expose the password to an external service.

There is no hard page limit. The tool processes all pages of both PDFs and concatenates the extracted text before comparing. For very long documents (100+ pages), text extraction may take a few seconds while the progress indicator shows. The comparison itself runs quickly once extraction is complete. For very large documents, consider splitting them by section using the PDF Splitter tool and comparing each section individually for faster, more focused results.

Yes, completely free. There are no usage limits, no account required, and no premium tier. You can compare as many PDFs as you need without any restriction. The tool uses Mozilla PDF.js, an open-source library, and runs entirely in your browser at no cost. No subscription, no per-comparison fee, and no watermarks on the results.

Unified View shows all lines from both documents in a single scrollable list — added lines (green, +), removed lines (red, -), and unchanged lines interleaved in sequence. This is the same format used by Git and most code diff tools. Side-by-Side View places the original document on the left and the modified document on the right in two columns, with rows aligned so corresponding lines appear at the same vertical position. Side-by-Side is generally easier to read for prose documents where you want to see both versions of a changed paragraph simultaneously.

Yes. The tool works on mobile browsers on iOS and Android. You can upload PDF files from your device's local storage or from cloud storage apps. For reading long diff outputs, a tablet or desktop provides a better experience, especially in Side-by-Side View, which benefits from wider screens. The upload zones are touch-friendly and the results panel is fully scrollable on mobile.