Add documentation for all PDF tools

docs/tools/fix-page-size.md

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+---
+title: Fix Page Size
+description: Standardize all pages in a PDF to a uniform size. Choose from standard paper sizes or set custom dimensions.
+---
+
+# Fix Page Size
+
+PDFs assembled from multiple sources often end up with mismatched page sizes -- some Letter, some A4, some completely non-standard. This tool forces every page to a single target size, scaling content to fit or fill as needed.
+
+## How It Works
+
+1. Upload a PDF file.
+2. Select a **Target Page Size** from the dropdown (A4, Letter, Legal, A3, A5, Tabloid, or Custom).
+3. Choose an **Orientation** -- Auto keeps each page's original orientation, or you can force Portrait or Landscape.
+4. Pick a **Content Scaling** mode: Fit (preserves all content, may add margins) or Fill (fills the page, may crop edges).
+5. Optionally set a **Background Color** for any added margins.
+6. Click **Fix Page Size**.
+
+## Options
+
+- **Standard sizes**: A4, Letter, Legal, A3, A5, Tabloid
+- **Custom dimensions**: Enter width and height in inches or millimeters
+- **Orientation**: Auto, Portrait, Landscape
+- **Scaling mode**: Fit or Fill (Crop)
+- **Background color**: Any color for margin padding
+
+## Use Cases
+
+- Normalizing a merged PDF where pages came from different scanners
+- Converting a US Letter document to A4 for international distribution
+- Preparing print-ready PDFs that require a consistent trim size
+- Fixing pages that are slightly off-standard (e.g., 8.49 x 11.02 inches instead of 8.5 x 11)
+
+## Tips
+
+- Use Fit mode to guarantee no content is lost. Fill mode looks cleaner but can clip edges.
+- Check your output with the Page Dimensions tool to confirm every page matches the target.
+- White is the default background color; switch to a dark color if your document has dark margins.
+
+## Related Tools
+
+- [Page Dimensions](./page-dimensions) -- inspect page sizes before and after fixing
+- [Compress PDF](./compress-pdf) -- reduce file size after standardization
+- [Crop PDF](./crop-pdf) -- manually crop pages instead of auto-scaling