Add documentation for all PDF tools

docs/tools/posterize-pdf.md

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+---
+title: Posterize PDF
+description: Split PDF pages into a grid of smaller pages for large-format printing across multiple sheets.
+---
+
+# Posterize PDF
+
+Divide each page of a PDF into a grid of smaller sub-pages. This is the tool for printing a single page across multiple sheets of paper, which you then tile together to create a large poster or banner.
+
+## How It Works
+
+1. Upload a PDF file.
+2. Set the number of **rows** and **columns** to define the grid (e.g., 2 rows x 2 columns splits each page into 4 sub-pages).
+3. Optionally specify a page range to limit which pages get posterized.
+4. The preview canvas shows a grid overlay on the current page so you can visualize the cuts.
+5. Navigate between pages to check the overlay on different pages.
+6. Click the process button. Each grid cell becomes its own page in the output PDF.
+
+## Features
+
+- Configurable row and column count for flexible grid sizes
+- Live preview with grid overlay showing exactly where pages will be divided
+- Page range filter to posterize only specific pages
+- Page-by-page navigation in the preview
+- Cached page snapshots for fast preview navigation
+- Dashed red grid lines clearly mark the cut boundaries
+
+## Options
+
+| Option         | Description                                  |
+| -------------- | -------------------------------------------- |
+| **Rows**       | Number of horizontal divisions (default: 1)  |
+| **Columns**    | Number of vertical divisions (default: 1)    |
+| **Page range** | Which pages to posterize (blank = all pages) |
+
+## Use Cases
+
+- Printing a poster-sized PDF across standard A4 or Letter sheets to assemble on a wall
+- Creating large banners from a single-page PDF design
+- Splitting oversized technical drawings into printable tiles
+- Making large-format prints without access to a wide-format printer
+
+## Tips
+
+- A 2x2 grid turns one page into 4 pages. A 3x3 grid turns one page into 9. Plan your grid based on how large you want the final assembled output.
+- The preview overlay helps you verify that important content does not fall on a cut line. Adjust rows/columns if needed.
+- For the reverse operation (fitting multiple pages onto one sheet), use [N-Up PDF](./n-up-pdf).
+
+## Related Tools
+
+- [N-Up PDF](./n-up-pdf)
+- [Divide Pages](./divide-pages)
+- [Split PDF](./split-pdf)