Add documentation for all PDF tools

docs/tools/fb2-to-pdf.md

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+---
+title: FB2 to PDF
+description: Convert FictionBook (FB2) ebook files to PDF in your browser. Free, private, no server upload.
+---
+
+# FB2 to PDF
+
+Convert FictionBook 2 (FB2) files into PDF documents. FB2 is an XML-based ebook format popular in Russia and Eastern Europe. While well-supported on certain platforms, it is virtually unknown on others -- converting to PDF makes your books readable everywhere.
+
+## Supported Formats
+
+| Format        | Extension | Notes                  |
+| :------------ | :-------- | :--------------------- |
+| FictionBook 2 | `.fb2`    | XML-based ebook format |
+
+## How It Works
+
+1. Click the upload area or drag and drop your FB2 files. Batch selection works for multiple files.
+2. Review the file list. Use the trash icon to remove individual entries, or **Add More Files** to include more.
+3. Click **Convert to PDF**. The PyMuPDF engine parses the FB2 XML structure and renders it as paginated PDF content.
+4. One file downloads as a PDF directly. Multiple conversions are bundled into `fb2-converted.zip`.
+
+## Options
+
+No configuration is needed. The converter handles the FB2 XML structure automatically, preserving chapter breaks, formatting, and any embedded images.
+
+## Use Cases
+
+- Reading FB2 ebooks on devices or apps that only support PDF
+- Printing Russian-language literature distributed in FB2 format
+- Archiving an FB2 library into a universally supported format
+- Sharing a book with someone whose reader does not support FictionBook
+
+## Tips
+
+- FB2 files are plain XML, so they tend to be small and convert quickly even in the browser.
+- If your FB2 files are compressed as `.fb2.zip`, extract them first before uploading.
+
+## Related Tools
+
+- [EPUB to PDF](./epub-to-pdf)
+- [MOBI to PDF](./mobi-to-pdf)
+- [XML to PDF](./xml-to-pdf)