Compress PDF rewrites the file so unused objects are dropped and the pages are packed more tightly. A scan that is already a set of JPEG images often shrinks little. A PDF exported with spare copies of the same font or image can shrink more. The page does not rasterize the document and does not run text recognition.
The download is still a PDF. It is not a ZIP and not a set of images. Merge PDF files if you need several documents in one file first. All tools