PDF Compression

I always scan copies of letters into my computer before I send them out. So I had a bunch of large PDF’s sitting around hogging my Dropbox space.

One day I found this blog post which claimed that simply running (in Bash) the commands
$ pdf2ps original.pdf temp.ps
$ ps2pdf temp.ps new.pdf

would decrease the file size. (The two commands are part of GhostScript, which I had installed on my Linux boxes anyways.) I couldn’t resist trying it — and miraculously, it worked. It generally decreases my scans by a factor of 10 (from 20MB to 2MB or so).

I have no clue why this works, although it probably has something to do with the fact that the PDF’s are scanned pages . Anyone care to enlighten me?