Document Attachments allow you to attach downloadable files to your document. This is helpful for appending supplemental attachments, larger file types, Excel spreadsheets, and any other content you’d like to attach to — but not include in the body of — your document.
Please be informed that you can attach a file in
document.draft status only.
You can attach up to 10 files, and no file can exceed 50MB.
More details please find in the HelpCenter topic.
Attachments of document transaction with few documents
Unfortunately, attachments creation is not allowed in a document transaction with few documents inside.