Prevent direct download of pdf document

Hello WPLMS team, I have recently bought WPLMS theme and it is very powerful. However I have a question about the attachments to unit, and more precisely pdf attachment. I wanted to attach a pdf document to a unit and then I followed the instructions disclosed in your documentation. However, when testing the unit, I noticed that the pdf document can be directly downloaded by a people who is not a student or a professor. Indeed, by WordPress default settings, all the documents are stored in the directory nameofthesite.com/wp-content/uploads/year/month/nameofthefile.pdf (if it is a pdf document). Then, if a people, who is not registered as student or professor, enters in the url adress navigator : nameofthesite.com/wp-content/uploads/year/month/, he can access to all the content of the directory and then download the pdf file. Is there a way to prevent such a thing, i.e. to prevent people who are not registered as students/professors to download pdf documents (which attached to a unit) by using the above described method ? Thank you in advance for your answer