This depends on the school the student is applying to. Each institution decides on its own what are acceptable ways to upload recommendation letters, so you will have to check and adhere to their instructions.

The graduate schools using our platform to receive applications can choose whether they want recommendation letters to be submitted as a PDF file or if they prefer their referees to fill in an online form - or even a combination of both! 

Usually, if the "PDF upload" feature is enabled, recommendation letters can be uploaded as a PDF file. If you don't see this option anywhere, then you will most probably only be able to fill out an online form and copy and paste the text of your recommendation letter into a special field.