I scanned a single document using Simple Scan, and it came out as a PNG (it defaults to JPG, but I changed to PNG to avoid pixelization).
I then converted at the command-line to PDF format (convert file.png file.pdf).
The resulting file was over 13MB, still too much for many email systems.
[Yes, I figured out since then that I could save it directly from Simple Scan to PDF simply by changing the extension. I still got a smaller file with the following method, which is also useful for PDFs that someone may give me.]
Someone suggested the following commands to shrink the file (from a terminal window):
pdf2ps file.pdf foo.ps
ps2pdf foo.ps file2.pdf
This worked really well, and reduced the PDF file size by more than 10x!