Navigate to the relevant place you wish to upload the documents.  Documents relating to an Applicant are usually kept in the documents section on the Contacts Sidebar.

If an applicant has applied for several vacancies, then an applicant could possibly have several documents for different vacancies, so if they are stored under the Contact, all the documents for that one person will be kept in one place.

Click on the Contacts Sidebar and click on the Documents tab (if you can not see it, you may need to click on the Related dropdown option)

Click on Upload and Navigate to where you saved the document and click OK