Depends entirely on the app the document is in. Most don't.
The most popular way around this is to upload your documents to DropBox or another cloud storage service. You can usually do this by using the Open In option to copy your document to the DropBox (or other) app, where you'll be prompted for name and folder info before it uploads.
Once the document(s) are in DropBox, you can generate share links for them, and paste as many as you like into the email.
Besides being a work around for multiple documents in an email, there are several other benefits to using share links instead of attachments.
- Speed: Emails are both sent and received faster.
- Flexibility: The recipient can choose to use mobile data or wait until they have a better connection to download the file.
- Repeatability: You can use the same share link to easily send the document to several people or several times.
- Integration: Quite a few apps have DropBox or other cloud services integrated, which means your document can be ready to share at any time.
And probably a couple of benifits I'm not thinking of. It's a different way of doing things, but once you get used to it it has it's charms.