diff --git a/crates/paperjam-docx/src/lib.rs b/crates/paperjam-docx/src/lib.rs
index fd0a358..b0fcb1f 100644
--- a/crates/paperjam-docx/src/lib.rs
+++ b/crates/paperjam-docx/src/lib.rs
@@ -74,12 +74,11 @@ impl DocumentTrait for DocxDocument {
     }
 
     fn save_to_bytes(&self) -> Result<Vec<u8>, Self::Error> {
-        let mut buf = std::io::Cursor::new(Vec::new());
-        self.inner
-            .clone()
-            .build()
-            .pack(&mut buf)
-            .map_err(|e| DocxError::Io(std::io::Error::other(e)))?;
-        Ok(buf.into_inner())
+        // Return the original input bytes. This crate exposes no mutation API,
+        // so a rebuild via `inner.build().pack()` would be lossy and — for any
+        // DOCX containing complex fields (TOC, PAGE, PAGEREF, HYPERLINK, …) —
+        // panics in docx-rs 0.4 (`RunChild::InstrTextString` is reader-only;
+        // its writer arm is `unreachable!()`). See bokuweb/docx-rs#750.
+        Ok(self.raw_bytes.clone())
     }
 }
diff --git a/py_src/paperjam/_any_document.py b/py_src/paperjam/_any_document.py
index f9af17b..ff700a8 100644
--- a/py_src/paperjam/_any_document.py
+++ b/py_src/paperjam/_any_document.py
@@ -133,9 +133,16 @@ def convert_to(self, format: str) -> bytes:
         return bytes(self._ensure_open().convert_to(format))
 
     def save(self, path: str | os.PathLike[str]) -> None:
-        """Save the document to a file."""
-        data = self.save_bytes()
-        with builtins_open(str(path), "wb") as f:
+        """Save the document to a file.
+
+        The target format is inferred from the file extension. If it differs
+        from the source format, the document is converted via :meth:`convert_to`;
+        otherwise the original bytes are written unchanged.
+        """
+        path_str = str(path)
+        target_ext = os.path.splitext(path_str)[1].lstrip(".").lower()
+        data = self.convert_to(target_ext) if target_ext and target_ext != self.format else self.save_bytes()
+        with builtins_open(path_str, "wb") as f:
             f.write(data)
 
     def save_bytes(self) -> bytes: