When you want to reduce the file size of a PDF document, this quick command using convert will shrink the original PDF file.
convert -density 150×150 -quality 60 -compress jpeg -colorspace Gray original.pdf new.pdf
This command is particularly useful against scanned documents, the jpeg quality will be 60% for 150dpi.
Converting an original 300dpi / color PDF to a 150dpi, greyscale PDF can reduce file size up to 50%. There will be some quality loss but in this way you can reduce file size enough to send scanned documents of dozens of pages via e-mail without using third-party services.