You're probably scanning to .pdf in a way that creates a bitmapped image of the page, rather than \ text (I do this quite often myself). In other words, the computer doesn't recognize the letters as letters, just as a series of dots. That's not an efficient way to digitally record and store text.
One solution is to use Optical Character Reader (OCR) software. Some probably came bundled with your printer. These programs look at the scanned page and convert the image into text. They're not perfect, but their errors are rare and almost always caught by spell-checkers. On the other hand, an OCR scan loses the graphics and formatting of the page, giving you just the text. Whether that's a problem or not depends on what you're scanning.
If OCR scanning isn't practical, try scanning at a lower resolution, and in black-and-white instead of color. That will shrink the file size considerably.
And if you just can't get the file small enough, see A Better Way to Send Large E-Mail 'Attachments'.
Read the original forum discussion at http://forums.pcworld.com/message/138427.