Cannot Send Email From Send Grid With Attachment 300 kb

I try to send email using sendgrid with attachment file (pdf). Its working good for attachment less than 250 kb. But when I try to send email with attachment file with size above 250 kb, I get error from send grid said, 552 5.2.3 your message exceeded google's size limit, but the attachment file only 300 kb. Can someone help me with this issue? thank you in advance.