A PDF can be one of the smallest file types, far less than a JPEG.
It has been the standard for sending to Press for a decade.
I suggest you look at your PDF settings. Distiller, the PDF maker of Adobe, has several presets and is almost infinitely variable.
If you are sticking to Acrobat, have a look at your 'Save as' settings.
Once again there are many variables, have a play and see what suits.