Our poster designer sent me a PDF of this year's Bloomsday poster. I thought the file was too large at 7.2MB and I wanted to reduce the file size without sig­nif­i­cant loss of image quality. I was unable to achieve this in Preview or Acrobat Reader, but Ghost­script did the trick, thanks to an answer on AskUbuntu:

gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 \
    -dPDFSETTINGS=/prepress -dNOPAUSE -dQUIET -dBATCH \
    -sOutputFile=output.pdf input.pdf

The results speak for themselves.