On the Docs & Contract tab click Add Document, give it a Name, choose the file (Allowed Types: pdf, jpg, jpeg, png) and Save. The document is listed with Name, Type, Upload Time and Preview / Download / Delete actions. Files you attach when sending the proposal email are stored here too.

Customers see all uploaded documents on a Documents tab of their proposal page (the tab only appears when there is at least one). Uploads and deletions are recorded in the Activity log. Errors: "Invalid file type. Please upload a valid file." for other formats and "File with the same name exists in the proposal" for duplicate file names — rename the file and try again.