There is no simple answer...
Reducing pdf files is avalible:
1) by using "Save As" method (this is helpful only some times),
2) by re-priniting it to pdf (using virtual pdf printer).
But in most cases to reduce pdf file size is needed to re-create the PDF file. To reduce PDF file size with this method, you can do it in 2 ways:
- by removing unwanted objects,
- by shrinking images.
Follow this useful link: http://stackoverflow.com/questions/210029/pdf-libraries
]. It's about libraries to manipulate PDF files.