Failed to load PDF document once sent by nodemailer attachments

Question

The email and attachment is successfully sent but the attachment failed to be opened/read!

Here is the code :

 let str = Buffer.from("Hello World").toString('base64')

console.log("str",str) //HelloWorlQ==

  let info = await transporter.sendMail({
    from: 'test5@gmail.com', // sender address
    to: "test@gmail.com", // list of receivers
    subject:"test", // Subject line
    attachments:[
        {   // encoded string as an attachment
        filename: 'text1.pdf',
        content: str,
        encoding: 'base64',
        contentType: "application/pdf"
    }
    ],
  });

Any clue please ?