Уменьшение размера PDF-файлв.

документация

Т.е. при вызове gs можно указать опцию -dPDFSETTINGS=configuration, где configuration может быть:

• /screen selects low-resolution output similar to the Acrobat Distiller (up to version X) “Screen Optimized” setting.
• /ebook selects medium-resolution output similar to the Acrobat Distiller (up to version X) “eBook” setting.
• /printer selects output similar to the Acrobat Distiller “Print Optimized” (up to version X) setting.
• /prepress selects output similar to Acrobat Distiller “Prepress Optimized” (up to version X) setting.
• /default selects output intended to be useful across a wide variety of uses, possibly at the expense of a larger output file.

На этой же странице чуть ниже про создание PDF

Ну и собираем всё вместе:

gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.3 -sColorConversionStrategy=UseDeviceIndependentColor -dPDFSETTINGS=/ebook -dNOPAUSE -dQUIET -dBATCH -sOutputFile=output.pdf input.pdf