Mail Merge for Gmail lets you easily include different file attachments for each email recipient. The add-on is integrated with Google Drive and you can use the built-in File Picker to attach one or more files from Drive into your emails.
While the Mail Merge sheet is active, select the row for which you need to insert the file attachments. Then go to Add-on, Mail Merge with Attachments, Add File and select the file from your Drive. You can repeat these steps to include another file attachment for the same recipient.
Attach the Same File to all Emails
This is useful for attaching different files for each recipient. However, if you wish to send the same document / file attachment to all merge recipients, there’s an easier way.
Open your Gmail draft and upload one or more files to the draft. Use this draft as a template for mail merge and the included attachment will be sent to all recipients.
Also, if you have included files under the File Attachments column of the Google Sheet, they’ll be attached as well to the email in addition to the common attachment that was included in the Gmail draft.