Compress Pdf Online already compresses the PDF files. So, there is not much you can do. What you can do is minimize the size of images in your document.
One solution for this is to create many images, compress them in the PDF format and use only one image for each page. You could use a number of image formats that is not supported by Apple in PDF. Here, I just want to talk about “iPad/iPhone/iPod” PDF file and I think there is a more simple solution... There is an option available “PDF Quality Enhancer” that increases the quality of PDF files. How to use “PDF Quality Enhancer”? You can easily check how many pages and images per page, by looking down to the top right of the document, at the image of the page. If the number of the image pages is less or the number of the image images is equal to 10, you can go to “PDF Quality Enhancer”. After that you have to use the “PDF Quality Editor” to improve the quality.