You can add certain types of files as attachments to Notes by dragging and dropping them. They'll sync via iCloud, so this can be an easy way to share files across two Macs. You can also access attachments you've added to notes from the icloud.com website; to download an attachment in a note, just double-click it.
[kirkmc adds: You cannot, however, access file attachments on iOS devices; they just show up as paper clips. Surprisingly, even photos don't show up in notes on my iPhone. Note syncing is abysmal, as is much of iCloud syncing; notes don't update correctly, or at all, some notes can't be saved from the web interface, and on iOS, it's a crap shoot whether notes display or not.]
Mac OS X Hints