First up open the proposals you wish to attach a document to. This can be a brand new proposal, and existing template, or a proposal you've already sent (you'll need to edit this last one).
Once are in the proposal editor:
- Click Add element, then Attachment
- Click on Click here to upload and select the document you want to attach
- You can add more than one attachment on the same row by clicking the More options arrow in the top right of the element, then Add column