Select the file manager, to have the file manager you will need to be a group manager

Create a new folder (1. below) and title it " Course_Resources" (2. below) then select upload (3. below)

The following screen will appear

Select add or drag and drop your PDF, finish by clicking upload. 

Once saved to a location  right click the PDF and select copy URL.

Then Refer to the Course Section type upload to external website, click here to learn how.