Add documentation for all PDF tools

docs/tools/background-color.md

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+---
+title: Background Color
+description: Change the background color of every page in your PDF while preserving all content on top.
+---
+
+# Background Color
+
+Set a solid background color behind all existing content in your PDF. The tool creates a colored rectangle on each page and layers the original content on top, so text, images, and graphics remain fully visible against the new background.
+
+## How It Works
+
+1. Upload your PDF file.
+2. Pick a background color using the color selector. The default is a light yellow (#FFFFCC).
+3. Click **Change Background Color** to process.
+4. The updated PDF downloads automatically.
+
+## Options
+
+- **Background Color** -- A standard color picker for choosing any solid color. Defaults to light yellow.
+
+## Features
+
+- Applies a uniform background color to every page
+- Preserves all original content (text, images, vector graphics) on top of the colored layer
+- Maintains original page dimensions
+- Works with documents of any page count
+
+## Use Cases
+
+- Adding a light cream or ivory background to give documents a softer, paper-like appearance
+- Color-coding document sets by applying different background colors to different categories
+- Creating visually distinct versions of the same document for different audiences
+- Adding a light background tint to improve readability when projecting documents on screen
+
+## Tips
+
+- Light, muted colors work best. A very dark background can make text hard to read unless the text is already light-colored.
+- If you want to change both the background and text color, apply the background color first, then use [Change Text Color](./text-color) on the result.
+- This tool works at the PDF structural level (not rasterization), so text remains selectable and searchable in the output.
+
+## Related Tools
+
+- [Change Text Color](./text-color)
+- [Invert Colors](./invert-colors)
+- [Adjust Colors](./adjust-colors)