It is not possible to add attachments to newsletters mostly because viruses are often sent via attachments. This causes many spam detectors to be more likely to reject emails that include files as attachments, and send those emails directly to the Spam folder. Attachments in newsletters can also affect the speed of sending and the deliverability of your campaigns.
While adding an attachment directly to your newsletter is not a possibility, there are some super simple ways to send external files to your readers.
Host the file on your website. Then you can include an external link in the body of your newsletter or a button block to download the file.
Use outside file hosting services such as Dropbox or Google Drive to host your file. Then you can also include an external link in the body of your newsletter or a button block to download the file from Dropbox or Google Drive.