Add documentation for all PDF tools

docs/tools/merge-pdf.md

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+---
+title: Merge PDF
+description: Combine multiple PDF files into one document with drag-and-drop reordering and page-level control.
+---
+
+# Merge PDF
+
+Combine multiple PDF files into a single document. You can merge entire files in a specific order, or switch to Page Mode for granular control over individual pages from every uploaded PDF.
+
+## How It Works
+
+1. Upload two or more PDF files by clicking the drop zone or dragging them in.
+2. Choose between **File Mode** and **Page Mode** using the toggle at the top.
+3. In File Mode, drag files to reorder them and optionally specify page ranges (e.g., `1-3, 5`) for each file. Leave the range blank to include all pages.
+4. In Page Mode, drag and drop individual page thumbnails to build your exact page sequence.
+5. Click **Merge PDFs** to combine everything into one file. The merged PDF downloads automatically.
+
+## Features
+
+- Two merge modes: File Mode for quick whole-file combining, Page Mode for per-page control
+- Drag-and-drop reordering in both modes with smooth animations
+- Page range syntax (e.g., `1-3, 5, 8-10`) lets you cherry-pick pages without switching to Page Mode
+- Lazy-loaded page thumbnails keep the UI responsive even with large documents
+- No limit on number of files or total page count
+- Preserves original PDF quality with no re-encoding
+
+## Options
+
+| Option          | Description                                                                             |
+| --------------- | --------------------------------------------------------------------------------------- |
+| **File Mode**   | Merge entire files in a custom order, with optional page range filters per file         |
+| **Page Mode**   | Visual grid of all pages from all files -- drag thumbnails to build your exact sequence |
+| **Pages input** | Per-file text field accepting ranges like `1-3, 5` to include only specific pages       |
+
+## Use Cases
+
+- Combining separate chapter PDFs into a single book or report
+- Merging a cover letter and resume into one application document
+- Pulling specific pages from multiple source documents into a consolidated file
+- Assembling scanned receipts or invoices into one archive PDF
+- Creating a presentation handout from slides across multiple decks
+
+## Tips
+
+- Use Page Mode when you need to interleave pages from different files rather than appending one file after another.
+- You can add more files after the initial upload using the **Add More Files** button without losing your current arrangement.
+- Page ranges are 1-indexed, so `1` refers to the first page of that file.
+
+## Related Tools
+
+- [Split PDF](./split-pdf)
+- [Organize & Duplicate](./organize-pdf)
+- [Alternate & Mix Pages](./alternate-merge)
+- [Delete Pages](./delete-pages)