By changing default pdf font you can reduce PDF size

our test results as follow

PDF before optimization = 1.4MB

PDF after optimization = 10KB

Please download attached file for example of invoice PDF optimized

Download

check line 576 in app/code/local/Mage/Sales/Model/Order/Pdf/Abstract.php its

 

we changed Lin Libertine Font with Times New Roman font

check 577

 

Hope this helps