Help:Reducing the file size of large PDF or DjVu scanned books
Some scanned PDF and DjVu books have unnecessarily large file size. It can be made smaller. This page talks about compressing such files.
Compressing PDFs with Cuminas technology edit
Cuminas offers a commercial, proprietary technology that claims to create PDFs that approach DjVu quality and size. How would it actually perform on PDF files present at Wikimedia Commons is unknown.