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docs/snippets/elixir/plugins/pdf_only_processor.md

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+<!-- snippet:skip reason="Elixir Rustler NIFs cannot host async Send + Sync + 'static Rust trait objects via callbacks; the BEAM actor-model boundary requires plugin work to live in the Rust core. The alef-generated Elixir trait_call macro additionally has a backslash/encoding bug (separate alef-codegen ticket). Custom plugins must be implemented in Rust." -->
+Custom post-processor implementation is not available in the Elixir binding. Post-processors must be implemented in Rust using the `PostProcessor` trait.
+
+To implement a PDF-only post-processor in Rust:
+
+```rust
+use kreuzberg::plugins::{Plugin, PostProcessor, ProcessingStage};
+use kreuzberg::{Result, ExtractionResult, ExtractionConfig};
+use async_trait::async_trait;
+
+struct PdfOnlyProcessor;
+
+impl Plugin for PdfOnlyProcessor {
+    fn name(&self) -> &str { "pdf-only" }
+    fn version(&self) -> String { "1.0.0".to_string() }
+    fn initialize(&self) -> Result<()> { Ok(()) }
+    fn shutdown(&self) -> Result<()> { Ok(()) }
+}
+
+#[async_trait]
+impl PostProcessor for PdfOnlyProcessor {
+    async fn process(
+        &self,
+        result: &mut ExtractionResult,
+        _config: &ExtractionConfig
+    ) -> Result<()> {
+        // Custom processing logic for PDF documents
+        Ok(())
+    }
+
+    fn should_process(
+        &self,
+        result: &ExtractionResult,
+        _config: &ExtractionConfig
+    ) -> bool {
+        result.mime_type == "application/pdf"
+    }
+}
+```
+
+Register this in your Rust initialization and Elixir will use it automatically during extraction.