langchain/document_loaders/bibtex.py/BibtexLoader/_load_entry
class BibtexLoader:    def _load_entry(self, entry: Mapping[str, Any]) -> Optional[Document]:
        import fitz

        parent_dir = Path(self.file_path).parent
        # regex is useful for Zotero flavor bibtex files
        file_names = self.file_regex.findall(entry.get("file", ""))
        if not file_names:
            return None
        texts: List[str] = []
        for file_name in file_names:
            try:
                with fitz.open(parent_dir / file_name) as f:
                    texts.extend(page.get_text() for page in f)
            except FileNotFoundError as e:
                logger.debug(e)
        content = "\n".join(texts) or entry.get("abstract", "")
        if self.max_content_chars:
            content = content[: self.max_content_chars]
        metadata = self.parser.get_metadata(entry, load_extra=self.load_extra_metadata)
        return Document(
            page_content=content,
            metadata=metadata,
        )