Function encrypt_pdf_with_qpdf 

pub fn encrypt_pdf_with_qpdf(
    pdf_data: Vec<u8>,
    password_options: &PasswordOptions,
    temp_dir: Option<&str>,
) -> Result<Vec<u8>>

Apply password protection using external qpdf tool (if available). This is a more robust solution for production use.

Prerequisites

• qpdf must be installed on the system
• Sufficient disk space for temporary files

Arguments

• pdf_data: The PDF data to encrypt
• password_options: Password and encryption settings
• temp_dir: Optional temporary directory (defaults to /tmp)

Returns

• Ok(Vec<u8>): Encrypted PDF data
• Err(Html2PdfError): If qpdf is not available or encryption fails