The PDF lecture slides of the lecturers are usually huge. The students then have to download huge PDFs and store them in e.g. OneNote in order to edit them. This fills the students' hard drive, and if a whole class downloads a 100 MB PDF at the same time, it can affect the speed and reliability of Moodle.
The new feature should compress all PDF documents by default, especially the integrated images. Lecturers forget to compress.