Add documentation for all PDF tools

docs/tools/flatten-pdf.md

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+---
+title: Flatten PDF
+description: Flatten interactive forms and annotations into static content. Makes form fields non-editable and burns annotations into the page.
+---
+
+# Flatten PDF
+
+Flattening merges interactive elements -- form fields, dropdowns, checkboxes, annotations, highlights, sticky notes -- directly into the page content. The visual appearance stays the same, but nothing is editable or extractable anymore. It is the PDF equivalent of printing a filled-out form and scanning it back in, except the quality stays perfect.
+
+## How It Works
+
+1. Upload one or more PDF files.
+2. Click **Flatten PDF**.
+3. A single file downloads directly. Multiple files produce a ZIP archive.
+
+The tool uses pdf-lib to flatten form fields (via `form.flatten()`) and a dedicated annotation flattener to burn annotations into the page stream. Both operations happen in a single pass.
+
+## What Gets Flattened
+
+- **Form fields**: Text inputs, checkboxes, radio buttons, dropdowns, signature fields
+- **Annotations**: Comments, highlights, strikethrough, underline, sticky notes, stamps, ink annotations
+- **Interactive elements**: Buttons, action triggers
+
+## What Stays Unchanged
+
+- Text content (remains selectable and searchable)
+- Images and graphics
+- Page layout and dimensions
+- Bookmarks and table of contents
+- Hyperlinks (unless they are annotation-based)
+
+## Use Cases
+
+- Locking a completed form so recipients cannot change the submitted answers
+- Preparing a PDF for archival where interactive elements are unnecessary
+- Fixing display issues caused by annotations rendering differently across viewers
+- Ensuring a signed document cannot be modified after the signature is applied
+- Reducing file size by eliminating form field definitions and annotation objects
+
+## Tips
+
+- Flatten after all edits are complete. There is no way to unflatten a PDF.
+- If you need to flatten and also strip metadata, JavaScript, and attachments, use the Sanitize PDF tool instead -- it includes flattening as one of its options.
+- Some PDFs report "no form found" -- this is normal if the document only has annotations and no form fields.
+
+## Related Tools
+
+- [Sanitize PDF](./sanitize-pdf) -- flatten forms plus remove metadata, scripts, and more
+- [Rasterize PDF](./rasterize-pdf) -- convert to image-based PDF for maximum flattening
+- [Font to Outline](./font-to-outline) -- flatten fonts to vector paths
+- [Encrypt PDF](./encrypt-pdf) -- lock the flattened PDF with a password