I am sure you have been there before, trying to submit an important form and your PDF with high resolution scans is too big to be accepted.
As you probably entered a lot of other data, you do not want to repeat the entire form if the page times out (which they seem to always do!).
This is an instant one-liner that substantially reduces the PDF size without much quality degradation (my PDF went from 8.9 MiB to 1.1 Mib):
gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 \
-dPDFSETTINGS=/ebook \
-dNOPAUSE -dQUIET -dBATCH \
-sOutputFile=output.pdf input.pdf
There are tons of options and parameters in Ghostscript to play with if you are interested, but the above will work for a start. Simply edit the input.pdf and output.pdf paths/files.