But how to attach multiple doc file in one email.
You're not going to be able to do this with the stock mail app, since it only looks in your photos section to attach multiple items. I still wish Apple would introduce a files.app someday in order to store non-photo documents.
Best workaround is to:
1. Use a cloud storage service to store and distribute documents (e.g. Dropbox)
2. Use an email app that allows for attaching documents from said cloud storage.
Off the top of my head, I know that the Mailbox app can sync with a Dropbox account to attach documents from.
Another solution is to use the cloud storage app directly, they will often let you select multiple items, then email them out all at once. However this doesn't help if you're trying to reply to an existing email with attachments (the above Mailbox+Dropbox solution works for this).
Basically, the closest you'll be able to get to using and emailing multiple documents on iOS is through the use of cloud storage applications.