When trying to shrink pdf size , target those components which have the greatest effect on the overall size of the document. Do not select the highest compression ratio, but a good one that will not affect the document quality too much. For the textual documents, optimization is capable of reducing the size significantly without lowering the quality. In case of scanned documents, reducing the resolution of images will have a greater effect. Always make sure to back up your data before compressing it, and after that open the file and see if everything is readable.