Well, you can stop trying. I did

Here’s the reason:  a pages document is not actually a regular file. It’s actually a “package”, a special kind of folder that appears as a single file when you look at it in finder. You can verify this by right clicking the pages document and then selecting “show package content” from the menu. It will show you the actual content of the pages “file”.
Since you can’t actually send a folder structure, or at least I haven’t seen an email client that can do that other than Apple Mail, then sending the pages “package” camouflaged as a file will not work.

If you use Apple Mail, it will automatically zip the files and send it, which is why attaching a pages document through Apple Mail works.

For all other clients a simple workaround is to compress the file into a zip file (right click the file and choose compress in leopard) and send the zip files.  This works every time.