When you’re busy in Pages and adding pictures to your document the file size increases dramatically, this is also reflected in the size of the PDF when you export it. There is however a very easy way to reduce the amount of MB by using the ColorSync Utility.
Just start that program, select ‘File’, ‘Open’. Choose your PDF, then select ‘File’, ‘Export’. In the dialog you now get choose as Quartz Filter ‘Reduce File Size’, hit save. You now have a PDF the fraction of the size of the original without any noticeable quality loss (depending on the original file).
I’ve been able to create a 1 MB PDF from an original of 60, while the pages document was 100 MB.