I had scanned in a document with the intention of emailing it. (For this I usually use PDFCreator which allows you to aggregate the results of several scans into a single PDF.) On this occasion, I had scanned all four pages of the document before realising that with, my current scanner settings, the resulting document would be about 12MB. So I was faced with the choice of either scanning them all again, or finding a way to reduce the size of the PDF. A quick Google turned up this link, which gave the following command line to use with Ghostscript:

gswin32c -sDEVICE=pdfwrite -dNOPAUSE -dBATCH 
-dPDFSETTINGS=/ebook -sOutputFile=C:newFile.pdf C:originalFile.pdf


The reason I had Googled for a Ghostscript solution was that I already knew I had it installed as part of Cygwin. (I always install Cygwin on any Windows machine I need to use regularly - mostly for the SSH client, but I usually do a full install just so that all those useful utilities are just there. After a bit of poking, I realised that instead of typing "gswin32c" I just needed "gs". The rest of the command worked just fine, and I ended up with a PDF of somewhat less than 2MB.

So here's a hat tip to the Ghostscript contributers over the years. Thanks. Isn't free software great?