There is software available to compress pdf's but none of it is free because PDF is such a busines oriented format... if you are doing this over and over it may be easier to just pay for something like this
http://www.cvisiontech.com/pdf_compressor_31.html We use it to compress documents on our document storage website at work and it is pretty good.
There is a free option that should work without having to use zip, rar or online storage...
Install CutePDF (the free version), it will install a software printer that you can print anything to and save as a pdf.
You will need to go to this page
http://www.cutepdf.com/Products/CutePDF/writer.asp and install both of the programs on the left, first the one that says free download, and then the one that says free convertor.
This will allow you to open the pdf you have in adobe reader, choose file, print, pick the cute pdf printer, go to properties > paper/quality > advanced > then set the print quality to something lower like 72DPI > hit OK > OK > OK and then save the document as pdf to your desktop. Go to the file and check the file size in the properties.
I have the real deal Adobe Acrobat 8 and have found that at the same quality settings cutepdf produces a smaller file.
Additionally I'm guessing ( so I might be wrong:-) that your PDF was created by scanning, if so this process just creates an image of each page and then saves the images as Tiffs inside a PDF file, that would explain the large file size. If your documents are text and your scanner software supports OCR (optical Character Recognition) you can use the OCR function to create a word doc and then print that to the cute pdf printer and save as pdf. Assuming your scanning mostly text this would produce an even smaller file than the way I described above. If you are scanning mostly images then you would be better off going with the method above.
-Matt