# Compare PDF — Technical Tool Documentation > **Summary:** Review two PDF versions side by side and highlight visual differences. > **Tool Route:** `/tools/compare.html` > **Visual Guide Route:** `/guides/compare-pdf/` > **Category:** Organize PDF > **Processing Architecture:** 100% Client-Side. Both files remain in local browser memory. --- ## 1. What the Tool Does Compare PDF renders matching pages from an original and modified document, then highlights differing pixels so you can review revisions without uploading either file. **Requirements & Constraints:** Two readable PDF files are required. The comparison is visual and page-based; it does not create a new PDF. --- ## 2. When to Use It Use **Compare PDF** whenever you need to perform client-side organize pdf operations without uploading confidential records to third-party cloud servers. **Ideal Scenarios:** - Working on personal identity records, banking statements, medical filings, or NDAs requiring 100% local confidentiality. - Fast offline or low-bandwidth processing directly in your web browser. - Universal cross-platform document operations on Windows, macOS, Linux, Android, or iOS. --- ## 3. Key Implemented Features - **Engine Architecture:** `pdf.js (dual rendering) + Canvas Pixel-by-Pixel Diff` - **Zero Server Uploads:** Processing executes directly within client browser memory. - **Instant Processing:** Fast execution powered by WebAssembly and native HTML5 APIs. - **Review Previews:** Visual feedback and progress indicators confirm results prior to saving. - **No Sign-up / No Watermarks:** Full functionality with zero paywalls or forced branding. --- ## 4. Supported Input Files - **Primary Input:** Two PDF documents (Original PDF and Modified PDF) - **Validation:** Files are validated in the browser prior to parsing. --- ## 5. Supported Output Files - **Output Format:** Interactive side-by-side on-screen visual difference audit - **MIME Types:** `application/pdf` (or appropriate image/text/zip formats). --- ## 6. File Size & Memory Limits - **Recommended Bounds:** Client RAM bound - **Technical Context:** Modern browser tabs enforce memory quotas (typically 1.5 GB to 2 GB per tab). PDFHub handles documents comfortably within standard browser memory limits. For files exceeding 150 MB or 1,000 pages, splitting into smaller sections is recommended. --- ## 7. Step-by-Step Instructions 1. **Open Compare PDF and choose the Original PDF file.** 2. **Choose the Modified PDF file and wait for both previews to load.** 3. **Select Compare Pages, then move through pages with the arrows.** 4. **Review the highlighted differences and page counts before acting on the result.** --- ## 8. Required User Inputs & Controls - **File Upload Area (`#drop` / `#fin`):** Click to open native file browser or drag and drop supported files. - **Tool Options (`#opts`):** Dynamic settings panel configuring parameters, page ranges, or compression levels. - **Execution Button (`Compare Pages (#goBtn)`):** Triggers client-side processing pipeline. - **Reset Button:** Clears loaded files and resets workspace to pristine state. --- ## 9. Processing Behavior & Execution Pipeline Renders corresponding pages from Document 1 and Document 2 onto dual canvases, extracts pixel image data arrays, compares RGBA values per pixel, and highlights visual discrepancies on an overlay canvas. --- ## 10. Expected Output & Download Verification - Upon successful execution, the interface triggers an automated browser download via `downloadBlob()` or opens a result review modal. - Users can verify file size, page count, and visual rendering before finalizing the download. --- ## 11. Practical Example Use Case Auditing contract revisions or engineering blueprints to spot subtle changes or unauthorized wording alterations. --- ## 12. Common Errors & Troubleshooting - **Issue:** If a file will not load, confirm it is a readable PDF and try a fresh copy. - **Issue:** If pages have different sizes, compare the content visually rather than relying on pixel counts. - **Avoid:** Selecting the same version twice hides meaningful changes. - **Avoid:** Expecting a downloadable redline file; this tool provides an on-screen comparison. - **Tip:** Comparing documents with completely different page counts or resolutions. Solution: Use matching document versions. --- ## 13. Privacy & Data Handling Architecture - **Local Processing:** 100% Client-Side. Both files remain in local browser memory. - **Browser Memory:** Document bytes reside temporarily in `ArrayBuffer` memory and are garbage collected when the browser tab is refreshed or closed. - **Cookies & Telemetry:** PDFHub sets zero tracking cookies, zero analytics beacons, and stores zero personal document data. --- ## 14. Frequently Asked Questions (FAQ) #### Q: Does this edit either PDF? **A:** No. It only renders and compares pages. #### Q: Can I compare more than two files? **A:** No. Use two files per comparison. --- ## 15. Related PDFHub Tools - [Merge PDF Documentation](merge-pdf.md) - [Split PDF Documentation](split-pdf.md) - [PDF to JPG Documentation](pdf-to-jpg.md) --- ## 16. Links & Resources - **Launch Live Tool:** [Open Compare PDF in PDFHub](../../tools/compare.html) - **Visual Tutorial:** [View Compare PDF Visual Guide](../../guides/compare-pdf/) - **All Tools Master Reference:** [All Tools Reference](../all-tools-reference.md) - **Master Documentation Index:** [Documentation Index](../documentation-index.md) --- ## 17. Visual Walkthrough & Interface Screenshots ### Step 1: Open Compare PDF - **Interface Control:** `Dual Drop Zones (#drop1, #drop2)` - **What You See:** The workspace features dual upload zones: Original PDF on the left, Modified PDF on the right. - **What to Do:** Open Compare PDF to highlight visual differences, edits, and pixel shifts between two documents. - **Practical Pro-Tip:** Compare PDF is an on-screen visual auditor that runs 100% locally in your browser. ![Compare PDF Step 1 - Open Compare PDF](../images/compare-pdf-step-1.png) ### Step 2: Upload Original Document - **Interface Control:** `Original File Browser (#fin1)` - **What You See:** The original document details and page count load into the left panel. - **What to Do:** Select the earlier or baseline PDF version into the first upload box. - **Practical Pro-Tip:** Comparing matching drafts with identical page sizes produces the clearest visual diff. ![Compare PDF Step 2 - Upload Original Document](../images/compare-pdf-step-2.png) ### Step 3: Upload Modified Document - **Interface Control:** `Modified File Browser (#fin2)` - **What You See:** The modified document loads into the right panel with its page count. - **What to Do:** Select the updated or modified PDF version into the right upload box. - **Practical Pro-Tip:** Both files must be readable PDFs. ![Compare PDF Step 3 - Upload Modified Document](../images/compare-pdf-step-3.png) ### Step 4: Review Side-by-Side Previews - **Interface Control:** `Preview Panes (.preview-wrap)` - **What You See:** Side-by-side previews show page 1 of each document. - **What to Do:** Confirm that both documents are aligned to corresponding sections. - **Practical Pro-Tip:** Verify page counts to see if any pages were added or removed in the revision. ![Compare PDF Step 4 - Review Side-by-Side Previews](../images/compare-pdf-step-4.png) ### Step 5: Click Compare Pages - **Interface Control:** `Compare Action Button (#goBtn)` - **What You See:** The visual diff engine rasterizes matching pages and runs pixel-by-pixel differential analysis. - **What to Do:** Click the primary "Compare Pages" button. - **Practical Pro-Tip:** All pixel analysis executes directly in WebAssembly inside your browser. ![Compare PDF Step 5 - Click Compare Pages](../images/compare-pdf-step-4.png) ### Step 6: Audit Highlighted Differences - **Interface Control:** `Diff Viewer (#diffViewer)` - **What You See:** Modified text, changed numbers, and shifted images are highlighted with vibrant diff colors. - **What to Do:** Inspect the highlighted color-coded difference overlays. - **Practical Pro-Tip:** Unchanged content remains neutral, allowing edits to stand out instantly. ![Compare PDF Step 6 - Audit Highlighted Differences](../images/compare-pdf-step-4.png) ### Step 7: Navigate & Audit All Pages - **Interface Control:** `Page Navigation Buttons (changePage)` - **What You See:** The comparison viewer shifts page by page, highlighting edits on each sheet. - **What to Do:** Use the left and right navigation arrows to audit differences across every page. - **Practical Pro-Tip:** Compare PDF is an interactive inspection tool; your original files remain unmodified. ![Compare PDF Step 7 - Navigate & Audit All Pages](../images/compare-pdf-step-4.png)