To reduce the size of the PDF one must first look into what is it that has made the PDF document bulky. When you look into this aspect you find that the factors that affect the size of the PDF file are the high resolution images embedded in it, the fonts that are used in the PDF document and also how these fonts are used in the document and finally the compression level of the document. Hence in order to reduce the size of the PDF document these are the elements you should look to modify.
You can actually find out which of these elements is occupying large space by using the PDF Optimizer. To do this you have to select the Advanced option and then navigate to PDF Optimizer from the drop down menu and navigate to Audit space usage. Herein you can find out the details of the space occupied by the elements mentioned above and having known this you can make changes to reduce the size of the file accordingly.
When it comes to managing the graphics, it is highly recommended that vector graphics are used instead of GIF or JPEG images. The advantage being the vector graphics take up less space. The added advantage is that the vector based images look better and can be scaled optimally. If the PDF document is created for the web then to reduce the size of the PDF you should use the RGB color mode and not the CMYK color for the latter makes the file bulkier.
Another important factor that should be borne in mind if you want to reduce the size of the PDF document is usage of fonts. The number of fonts used should be minimal and it is also desirable to subset fonts rather than embedding. There are many Free PDF compressors available which can be downloaded and used to help you reduce the size of your PDF files and generate small size files.