It seems that sending files other than photos from the Windows Phone 8 isn’t quite so easy as it maybe should be, the insert feature is great for adding a photo but no go for files.
Skydrive is the place WP8 seems to want to store files and it is tightly integrated with the whole OS which is a good thing however even from within Skydrive the ability to send a file rather than a link is also not supported.
To Google I go and here is what I found.
SkyDrive Office Documents
For sending an office document you have stored on SkyDrive:
Open the file from the office hub. Edit or view, etc. Then click the options on the bottom right (…) and click (save as). Save it to your phone. Once it is done then click the options on the bottom right (…) and select share. Select your email source and it will launch an email with the actual document attached.
To view a pdf locally on your phone after it is downloaded from an email, it is actually stored in the pdf reader app. You can’t send it, which is a shame, but that is where it is stored after it is viewed from the email.
The post here covers the issues and some resolutions
When you open the app the first time, you will start with the Welcome page. After tapping the next button, you have to log in with your Microsoft account to the SkyDrive. If you managed to sign in o your SkyDrive, the Settings page will appear, where you have to provide a valid SMTP credential (the username and password are the same as you have provided in the previous step, but you can specify a different one as well). After you tap on the save button, you can compose your email. By tapping on the attach button, the content of the root SkyDrive folder should appear (if not, please use the refresh button ).
While I can see the whole point of this, keep your files stored on Skydrive and link to them however the rest of the world isn’t quite ready yet for links and needs attachments, so this lack of functionality is a bit of a pain. So until the rest of the world catches up Mail Plus might be the answer.