Under 'CRM,' select 'Vendors.'

Click on the supplier that you want to add a document to then click on the blue script at the bottom.

 Click on the plus button.

This will the open the add a document screen. Click on the folder on the left hand side which will then open the documents area.

Choose the document that you want to associate to the supplier.

Your document has now been attached to the supplier. Click on the green cross at the bottom right hand side to complete.