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