feat: add pdftotext quality heuristic with tests

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/main.rs

@@ -292,6 +292,18 @@ fn validate_pdf(bytes: &[u8]) -> anyhow::Result<()> {
 // Conversion
 // ---------------------------------------------------------------------------
 
+/// Check if pdftotext output is usable: long enough and mostly readable text.
+fn pdftotext_output_is_good(text: &str) -> bool {
+    if text.len() < 500 {
+        return false;
+    }
+    let printable = text.chars().filter(|c| {
+        c.is_alphanumeric() || c.is_whitespace() || c.is_ascii_punctuation()
+    }).count();
+    let ratio = printable as f64 / text.chars().count() as f64;
+    ratio > 0.8
+}
+
 /// Write PDF bytes to a temp file, run marker_single, and return the
 /// resulting markdown.
 fn convert_to_markdown(pdf_bytes: &[u8]) -> anyhow::Result<String> {
@@ -342,3 +354,25 @@ fn find_markdown_file(dir: &std::path::Path) -> anyhow::Result<String> {
     }
     bail!("no .md file found in marker output")
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn good_text_passes_quality_check() {
+        let text = "This is a normal academic paper abstract. ".repeat(20);
+        assert!(pdftotext_output_is_good(&text));
+    }
+
+    #[test]
+    fn short_text_fails_quality_check() {
+        assert!(!pdftotext_output_is_good("too short"));
+    }
+
+    #[test]
+    fn garbled_text_fails_quality_check() {
+        let garbled = "\u{fffd}\u{25a0}\u{2588}\u{2591}".repeat(200);
+        assert!(!pdftotext_output_is_good(&garbled));
+    }
+}