You can have a look at the following link which generally describes that how to download a file on a button click from a folder.
Download a file on Button Click
Further you can change the content Type as per you requirement to make this code work for different file types.
Further for the list of Content/MIME Types.
You can have a look at this link.
List of Content Types.
and if we talk to the point.
You have to change the content type to
application/pdf
for PDF download.
Hoe it will help. :)