Supplier Frontend - Supplier - Creating Downloadable Products
To add a Downloadable Product, please go to the Vendor Portal and click on "Add a Product" in the navigation bar.
Now you can easily choose Downloadable Product in Product Type field.
When in the "Add Product" form, on the right side, the supplier will find fields to upload the file on the bottom of Product Categories.
The other fields of the new product are described in Supplier Frontend - Adding Products article.