There are four ways to upload documents into DOKKA, and one of them is uploading via email. Every company has a unique email address. Send invoices to that address, and they will be automatically uploaded into DOKKA. You'll be able to see them in the Inbox folder in your Workspace.
The default address is a string of numbers (accountants like numbers). You can change it to something more meaningful. Here's how:
Click on the cogwheel on the top right side of the screen and choose Settings:
Enter your choice of address in the email field and click Save.
There are many other options to configure at this screen, and you can read all about them in this article.