Researching implies producing lots of PDF’s. Sometimes these PDF’s are exported without much control and become several MB large, which is inconvenient to send out to some else (e-mail) or even to the cloud or a printer. To deal with this issue, a common solution is to down sample the images in your PDF to a lower DPI value (reducing the number of pixels per square inch). If the target of the document is a handout paper (A4 or Letter size), 300 dpi is usually enough. But you might want to go even lower if the purpose is only screen-reading (150 or 150 dpi).
The industry standard software to fulfil this task is Adobe Acrobat or Adobe Distiller, but there are free options. One of those is using Ghostscript, a free set of tools widely used in the scientific community. After you install, run this command in OS X Terminal, replacing “input.pdf” with your own target file. Remember that drag ‘n dropping a file into the terminal will immediately write its path. The output will be in your home folder under the name “downsampled.pdf”.
gs \ -o downsampled.pdf \ -sDEVICE=pdfwrite \ -dDownsampleColorImages=true \ -dDownsampleGrayImages=true \ -dDownsampleMonoImages=true \ -dColorImageResolution=72 \ -dGrayImageResolution=72 \ -dMonoImageResolution=72 \ -dColorImageDownsampleThreshold=1.0 \ -dGrayImageDownsampleThreshold=1.0 \ -dMonoImageDownsampleThreshold=1.0 \ input.pdf
This code was borrowed from Kurt Pfeife.