How to decrease the PDF file size?
One possibility is reducing the PDF Driver Resolution.
How to do that:
MicroStation -> File -> Print-> the Print Dialog Box opens ->File -> Select Bentley Driver -> select the Bentley driver pdf.pltcfg -> open -> the Print Dialog Box is open
Print Dialog Box ->File -> Edit Printer Driver Configuration -> Base Properties TAB ->Advanced->
Driver Resolution = 600.600
This is the place to change the driver Resolution
After you have change the Driver Resolution go to File -> Save; File -> Exit.
The Print Dialog Box is now open go to File -> Reload printer Driver Configuration.