# Merge PDF — Technical Tool Documentation > **Summary:** Combine multiple PDFs into one document in the selected file order. > **Tool Route:** `/tools/merge.html` > **Visual Guide Route:** `/guides/merge-pdf/` > **Category:** Organize PDF > **Processing Architecture:** 100% Client-Side. No network calls or server transmissions. --- ## 1. What the Tool Does Merge PDF copies every page from two or more selected PDFs into one document, following the order shown in the file list. **Requirements & Constraints:** At least two readable PDF files are required. The current list supports removal but not drag reordering. --- ## 2. When to Use It Use **Merge 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-lib (PDFDocument.create, copyPages, addPage)` - **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 or more PDF files (.pdf) - **Validation:** Files are validated in the browser prior to parsing. --- ## 5. Supported Output Files - **Output Format:** Single merged PDF file (merged.pdf) - **MIME Types:** `application/pdf` (or appropriate image/text/zip formats). --- ## 6. File Size & Memory Limits - **Recommended Bounds:** Client RAM bound (~150-200 MB combined input recommended) - **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. **Select or drop two or more PDFs.** 2. **Review the file list and remove any incorrect file.** 3. **Confirm the list order is the desired page order.** 4. **Select Merge PDFs and review the combined download.** --- ## 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 (`Merge PDFs (#mergeBtn)`):** Triggers client-side processing pipeline. - **Reset Button:** Clears loaded files and resets workspace to pristine state. --- ## 9. Processing Behavior & Execution Pipeline Reads each file into an ArrayBuffer, parses PDF catalogs via PDFDocument.load, creates a new blank PDFDocument, iterates through each source document copying page references into the target document, and serializes the binary structure with doc.save(). --- ## 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 Combining separated quarterly tax reports, receipts, and an executive summary cover page into one cohesive dossier for filing. --- ## 12. Common Errors & Troubleshooting - **Issue:** If one file fails, remove it and test that PDF separately in Repair PDF. - **Issue:** If the browser blocks multiple downloads elsewhere, allow downloads for the site. - **Avoid:** Selecting fewer than two files. - **Avoid:** Assuming filenames are automatically sorted alphabetically. - **Tip:** Encrypted or password-protected PDF loaded without permissions (fails with PasswordRequired error). Solution: Unlock the PDF first with Unlock PDF. - **Tip:** Selecting a single file (Merge button remains disabled). Solution: Add at least two PDF documents. - **Tip:** Corrupt PDF stream in one file. Solution: Run the corrupt file through Repair PDF before merging. --- ## 13. Privacy & Data Handling Architecture - **Local Processing:** 100% Client-Side. No network calls or server transmissions. - **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: Can I reorder files after selecting them? **A:** The page currently merges in the displayed selection order. #### Q: Are PDFs uploaded? **A:** The merge happens in the browser. --- ## 15. Related PDFHub Tools - [Split PDF Documentation](split-pdf.md) - [Compress PDF Documentation](compress-pdf.md) - [Organize PDF Documentation](organize-pdf.md) --- ## 16. Links & Resources - **Launch Live Tool:** [Open Merge PDF in PDFHub](../../tools/merge.html) - **Visual Tutorial:** [View Merge PDF Visual Guide](../../guides/merge-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 Merge PDF - **Interface Control:** `Upload Drop Zone (#drop)` - **What You See:** The drag-and-drop box displays "Drop PDFs here or click to upload" and accepts multiple files. - **What to Do:** Open the Merge PDF tool and locate the central upload area. - **Practical Pro-Tip:** Make sure your browser has permission to select files from your local storage. ![Merge PDF Step 1 - Open Merge PDF](../images/merge-pdf-step-1.png) ### Step 2: Select PDF Documents - **Interface Control:** `File Browser (#fin)` - **What You See:** Your operating system file chooser opens, allowing you to highlight multiple PDF documents. - **What to Do:** Click the upload zone and select two or more PDF files you wish to merge into a single document. - **Practical Pro-Tip:** You can select multiple files at once by holding Ctrl (Windows) or Cmd (Mac) in the file picker. ![Merge PDF Step 2 - Select PDF Documents](../images/merge-pdf-step-2.png) ### Step 3: Check Loaded Documents - **Interface Control:** `Document List (#flist)` - **What You See:** Each selected file shows its PDF icon, file name, formatted file size in MB, and a removal button. - **What to Do:** Review the file cards that appear in the document queue. - **Practical Pro-Tip:** If an unwanted document was accidentally added, click its red "×" button to remove it. ![Merge PDF Step 3 - Check Loaded Documents](../images/merge-pdf-step-3.png) ### Step 4: Arrange Document Order - **Interface Control:** `Queue Reordering (#flist .file-item)` - **What You See:** The list reflects the exact sequence that pages will follow in the final merged document. - **What to Do:** Check the sequential order of files from top to bottom. - **Practical Pro-Tip:** The document at the top of the list will become the first pages of your new combined PDF. ![Merge PDF Step 4 - Arrange Document Order](../images/merge-pdf-step-4.png) ### Step 5: Click Merge PDFs - **Interface Control:** `Merge Action Button (#mergeBtn)` - **What You See:** The button activates once two or more valid PDFs are loaded in the queue. - **What to Do:** Click the primary "Merge PDFs" button located below the document queue. - **Practical Pro-Tip:** Merging occurs 100% locally on your computer—no files are sent to remote servers. ![Merge PDF Step 5 - Click Merge PDFs](../images/merge-pdf-step-4.png) ### Step 6: Wait for Processing - **Interface Control:** `Progress Bar (#prog, #pbar)` - **What You See:** The progress bar advances from 10% to 100% and displays "Merging PDFs…". - **What to Do:** Observe the animated progress bar and status indicator as pages are copied. - **Practical Pro-Tip:** Large documents with hundreds of pages may take a few seconds to compile in memory. ![Merge PDF Step 6 - Wait for Processing](../images/merge-pdf-step-4.png) ### Step 7: Review Merged Document - **Interface Control:** `Review Section (#generatedPreview)` - **What You See:** A mandatory review panel appears with an embedded PDF preview and the default file name. - **What to Do:** Inspect the generated preview frame to confirm that pages combined successfully. - **Practical Pro-Tip:** You can customize the output file name directly in the file name input box before saving. ![Merge PDF Step 7 - Review Merged Document](../images/merge-pdf-step-4.png) ### Step 8: Download Combined PDF - **Interface Control:** `Download Dialog (#dlModal .btn-success)` - **What You See:** The ready-to-download modal displays the file name, final size, and green download button. - **What to Do:** Click the green "Download" button in the download dialog. - **Practical Pro-Tip:** Check your browser downloads folder for your new combined document. ![Merge PDF Step 8 - Download Combined PDF](../images/merge-pdf-step-4.png)