Thanks for reaching out to the SmarterTools Community. I'm not sure about google docs. But I know that you can do this with files from google drive. To do this you first need to enable cloud connections. This is in the features tab on the domain.
After this, you need to go to webmail. Then click settings->connectivity->New Provider in the Cloud Storage Connections card. After this just follow the prompts and allow the unverified app.
Then to access the files compose a new email. Click Actions(Button with three dots)->Link File->Google Drive
Then just click the file and click attach.
After clicking attach you should see something like this in the email body.
That's the link to the Google Drive file.
Please let me know if this helps.
Technical Support Specialist