How to reduce file size of PDF in customised way?

Adobe Acrobat is installed on my Macbook Pro. I am considering buying it. When I convert a Word document or a Powerpoint presentation into PDF it almost double the file size. I can't find any menu where I can reduce the file size in customised manner. There is 'reduce file size' button, but despite clicking on that the file size remains large and does not allow any customised reductions. What is your guidance on this? Thanks.