Add documentation for all PDF tools

docs/tools/pdf-to-pdfa.md

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+---
+title: PDF to PDF/A
+description: Convert standard PDFs to PDF/A format for long-term archiving. Supports PDF/A-1b, PDF/A-2b, and PDF/A-3b conformance levels.
+---
+
+# PDF to PDF/A
+
+PDF/A is an ISO-standardized subset of PDF designed for long-term digital preservation. This tool converts your standard PDFs into PDF/A-compliant documents using Ghostscript running directly in your browser.
+
+## How It Works
+
+1. Upload one or more PDF files.
+2. Select a **PDF/A Version** from the dropdown.
+3. Optionally enable **Pre-flatten PDF** for complex files that fail normal conversion.
+4. Click **Convert to PDF/A**. Multiple files produce a ZIP archive.
+
+## PDF/A Versions
+
+| Version      | Restrictions                               | Best For                                          |
+| ------------ | ------------------------------------------ | ------------------------------------------------- |
+| **PDF/A-1b** | Strictest. No transparency, no layers.     | Maximum compatibility with older archival systems |
+| **PDF/A-2b** | Allows transparency and JPEG2000.          | General-purpose archiving (recommended)           |
+| **PDF/A-3b** | Allows embedded attachments of any format. | Archiving documents with source data files        |
+
+PDF/A-2b is selected by default and works for the vast majority of archiving needs.
+
+## Pre-flatten Option
+
+Some PDFs contain features that prevent clean PDF/A conversion -- unusual font encodings, complex transparency stacks, or broken internal structures. Enabling **Pre-flatten** rasterizes the PDF at 300 DPI before conversion, which guarantees compliance at the cost of making text non-selectable. Use it as a fallback when the standard conversion produces errors.
+
+Pre-flattening requires the PyMuPDF WASM engine.
+
+## Use Cases
+
+- Meeting government or legal archival requirements (many jurisdictions mandate PDF/A for court filings)
+- Submitting documents to institutional repositories or digital libraries
+- Preserving business records where the original software may not exist in ten years
+- Preparing invoices for e-invoicing standards that require PDF/A-3b with XML attachments
+
+## Tips
+
+- Try conversion without pre-flatten first. Only enable it if you get an error.
+- PDF/A embeds all fonts, so output files are often larger than the originals.
+- Validate the output in a dedicated PDF/A validator if strict compliance is required for your workflow.
+
+## Related Tools
+
+- [Compress PDF](./compress-pdf) -- reduce the file size after conversion
+- [Flatten PDF](./flatten-pdf) -- flatten forms before converting to PDF/A
+- [Remove Metadata](./remove-metadata) -- strip metadata before archiving