Right now bid documents in Bonfire are only available for download one file at a time. This is intentionally set up so that Project Owners have insight internally as to which Vendors have downloaded which documents. This is especially important if any documents are added after the project opens as the Project Owner has the ability to view traction on if that documentation is being viewed by Vendors.
Project files can be downloaded under the Supporting Documentation heading from the opportunity page.
To find out more about accessing these documents, see How do I find the documents for the opportunity?