Attachments, (both internal and for clients) can be added from the Proposal Settings. (add link)
After filling out the Required Fields in Proposal Settings, set the "Show All Options" toggle to ON:
Scroll down to find "File Attachments". From here you can drag and drop the attachment from your computer into the dotted area.
Or select "Click here to upload"
After the attachment has been added, you can use the ON/OFF toggle to make the attachment visible to clients. Leaving the toggle ON will make the attachment visible to clients
Leaving the toggle ON will make the attachment visible to clients
Setting the toggle to OFF will make this an internal attachment, only visible to you and your team.