How can I reduce the file size in Adobe Reader 9?

In Acrobat, open a PDF file.
-Choose Document > Reduce File Size.
-Select Acrobat 8.0 And Later for file compatibility, and click OK.

When you choose the compatibility level, be aware that the newer the version of Acrobat that you choose, the smaller the file. If you choose compatibility with Acrobat 9, however, you should be sure that your intended audience does indeed have version 9 installed.

Name the modified file. Click Save to complete the process.

It is always a good idea to save a file using a different name so that you don't overwrite an unmodified file.
Acrobat automatically optimizes your PDF file, a process that may take a minute or two. Any anomalies are displayed in the Conversion Warnings window. If necessary, click OK to close the window.
Minimize the Acrobat window. View the size of the reduced file. The file size is smaller.

You can repeat steps stated above using different compatibility settings to see how they affect file size. note that some settings might actually increase the file size.
PDF Optimizer offers more opportunities to control quality trade-offs.
Choose File > Close to close your file.