Pages, and Other iWork documents are not single documents like in other programs. They are actually folders when viewed by the finder or file attach dialogs.
You need to compress the file, as mentioned by someone else, since a Zip file is not viewed in this way and can be attached.
Personally, if your sending Mac to Mac, I prefer Disk images. They do not expand into a new folder or item on the desktop. They open in the sidebar as disks. You can then drag n drop the file where you want it, eject the disk image, and delete the original dmg file.
Jason