Adjust size is only meant to adjust the size of an image, such as a JPEG or GIF. There's really no way to adjust a PDF to make it smaller other than perhaps reducing the size of images in the PDF before it's created.
There are a few tools that might allow the user to separate PDFs by pages. That might be helpful in cases where there's a maximum single file size that can be uploaded. I have a tool called Combine PDFs. Despite its name, it can be used to split up PDFs. There are other tools that work similarly.
http://www.monkeybreadsoftware.de/Software/CombinePDFs.shtml