from pypdf import PdfReader


def parse_pdf(file_path: str) -> str:
    """Parse a PDF file and return its text content.

    Text is collected from each page and concatenated with newlines between pages.
    Empty PDFs or corrupted files raise ValueError.
    """
    try:
        reader = PdfReader(file_path)
    except Exception as exc:
        raise ValueError(f"Invalid PDF file: {exc}") from exc

    texts = []
    try:
        for page in reader.pages:
            text = page.extract_text()
            if text:
                # Normalize line endings and trim unrelated whitespace
                texts.append(text.strip())
    except Exception as exc:
        raise ValueError(f"Failed to extract text from PDF: {exc}") from exc

    return "\n".join(texts).strip()
