Hello, PDF submission is same as document sharing . we can put our link inside the pdf which are effective and also come in SERP.
tbh never used it but i can see it as inserting backlinks in your PDF files and then submitting them to a website.
PDF submission takes place when you have a E book or PDF where yo can hyperlink your articles into it which is ultimately linked with your websites. PDF sometimes can be used as link bait also provided it will have to be very exciting.