If you're willing to learn how to use the commandline, then there's a free program called GhostScript that might work for you.
gswin32c.exe -sDEVICE=pdfwrite -dPDFSETTINGS=/ebook -sOutputFile="outfile.pdf" -dNOPAUSE -dBATCH "infile.pdf"
Replace /ebook with /screen for even smaller files (however, the smaller the file, the lower the quality). infile.pdf is the name of the that is too big, outfile.pdf is the new file that will be produced.
Whether the file gets smaller depends on many things but this is a starting point.
There are a lot of details about this process that you can read here:
GhostScript can be downloaded from here:
To use the above commandline, you might need to change your system path to include the folder where GhostScript is installed.
If you don't want to use commandline, then there is another free program that may also help you. It is called PDFCreator. You can print your PDF document into another PDF file and compress it further. You will need to change the PDF Format Options when you get to the Save screen (after selecting PDFCreator as printer, click print, then click Save Options on the next screen. Under Formats -> PDF, select ebook or screen).
I prefer the commandline version because it produces better results but you can certainly use the PDFCreator method too.
PDFCreator is available from here:
When installing, pay attention to the install options. Uncheck anything that has to do with a "toolbar". You probably don't want to install that. This is also why I prefer the commandline program, you don't have to deal with this stuff.