You should enable the option in Acrobat Distiller with the following steps.
Open the Acrobat Distiller. Click Settings > Job Options from the menu bar.
In the Job Options windows, switch to the Compression tab. For better quality (higher resolution) output, increase the dpi value for the appropriate groups. For smaller file size, decrease the dpi value. Actually, the default setting is suitable to build an efficient PDF file for balanced file size and quality.
After all, save the changes by pressing OK or Save As for the Acrobat to take effect on the next process.