The CRM has the option to send this estimate to the customer which is related. Also, the PDF estimate will be attached to the email. If you want to do not attach the PDF estimate simple uncheck the checkbox Attach Estimate PDF
After sending the estimate to the customer if you want to send again the estimate there will be another email content. Because the initial estimate is already sent. Now the customer maybe want the estimate again and in all cases, this content will be different. Go to Setup-> Email Templates -> Estimate Already Send to Client to edit the email content when sending the estimate to your customer for the second time.