There are various methods that one can use to compress a PDF file. First, be sure to check out the audit space usage tool under the PDF optimizer utility. This will give you a clear idea of what occupies how much space and what you need to reduce in order to optimize your PDF. Most of the times you will see that 9 or so different categories, only a few like content streams, structure info, bookmarks, images and fonts take up the most space. These are the ones you need to plan to reduce in size to optimize your PDF document.
The next stage: Using PDF optimizer to compress images
Various options are available to compress PDF images. Depending on whether your images are color, grayscale or monochrome, you can use various types of downsampling and compression methodologies. Downsampling may include average, subsampling or bicubic. Compression methods may be jpeg, zip, run length, jbig2 and others as well, depending on the color types of the images. You can also set the type of compression to use depending on pixels per inch ratios, as well as set the quality of the output and also preferred version to make the output compatible with.
The other step: compress fonts using PDF optimizer
Fonts can each add 40 kilobytes or more to the PDF file size. While making a PDF file, designers are often not conscious about how the use of too many fonts can drastically increase the size of a PDF document. These are various fonts both for text as well as for bullets etc, and one should try and use as few fonts as possible, with a well thought out plan beforehand. The PDF optimizer can then be used to still reduce the number of embedded fonts and compress the PDF file even further.