Attempt to download PDF of a CrossRef paper.
Args:
paper_id: CrossRef DOI (e.g., '10.1038/nature12373').
save_path: Directory to save the PDF (default: './downloads').
Returns:
str: Message indicating that direct PDF download is not supported.
Note:
CrossRef is a citation database and doesn't provide direct PDF downloads.
Use the DOI to access the paper through the publisher's website.
Connector