Add documentation for all PDF tools

docs/tools/pdf-to-greyscale.md

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+---
+title: PDF to Greyscale
+description: Convert a color PDF into a greyscale (black-and-white) version. Produces a new PDF with all color removed from every page.
+---
+
+# PDF to Greyscale
+
+Converts a color PDF into a greyscale version. Each page is rendered, desaturated, and re-embedded as a JPEG image in a new PDF. The output preserves the original page dimensions.
+
+## How It Works
+
+1. Upload a PDF by clicking the drop zone or dragging a file onto it.
+2. Click **Convert** to process the file.
+3. A new `greyscale.pdf` downloads with all pages converted to black and white.
+
+Under the hood, each page is rendered at 2x scale, the pixel data is run through a greyscale filter, then re-encoded as a JPEG at 90% quality and placed in a fresh PDF document.
+
+## Options
+
+This tool has no configurable options. The greyscale conversion uses a standard luminance formula and JPEG quality is fixed at 90%.
+
+## Output Format
+
+- A single `greyscale.pdf` file containing all pages as greyscale JPEG images.
+
+## Use Cases
+
+- Preparing documents for black-and-white printing to save on color ink costs.
+- Meeting submission requirements for legal or government filings that mandate greyscale.
+- Reducing file size on color-heavy PDFs (JPEG greyscale images are smaller than color).
+- Creating a consistent visual style across documents with mixed color and B&W pages.
+
+## Tips
+
+- Because each page is rasterized and re-embedded as JPEG, the output is image-based. Text in the original PDF will no longer be selectable or searchable. If you need searchable text afterward, run the output through the OCR tool.
+- For more control over color adjustments (brightness, contrast, saturation), look at the Adjust Colors tool or the Greyscale node in the [PDF Workflow Builder](./pdf-workflow).
+
+## Related Tools
+
+- [PDF to JPG](./pdf-to-jpg)
+- [PDF to PNG](./pdf-to-png)
+- [PDF Workflow Builder](./pdf-workflow)