Q: how can you reduce the file size for email

If you scan and use Preview to save as PDF, and then use a website such as http://smallpdf.com/compress-pdf you can then attach the small PDF that results in your download folder.   Mind you, you will probably want to rename the PDF before attaching it, as some people's e-mails don't handle long file names well.

Your scanner may not be performing compression or other steps that might reduce the resulting image size, or what you're getting are not image scans.   Or just push the higher-resolution files to Dropbox or Spideroak, or another file-sharing service.