If you want to insert or embed an ad-hoc PDF file into your proposal, it is very simple. Just click on the "+ Insert" button and select Attach PDF File from the drop-down:
A form will apper asking you to provide a title, description and upload the PDF itself. Also you have the option to embed the pdf file inside your proposal or not. If you choose to embed the PDF file it will physically be added as additional pages to your proposal. If you choose not to embed, the PDF will be a dynamic link that the user can click to view that file.
You can attach as many as you like in every section of the proposal.