Open the document within Drawboard PDF that you wish to share.
In the main menu on the left, tap on More (represented by the '...' icon).
Select Share from the list of options.
Note: Only compatible apps installed from Microsoft Store app will be shown.
Note 2: This function only allows on PDF to be sent at a time to other apps, this means you cannot attach multiple PDF documents to the same email.
Sharing a document via Email
You can email documents directly from the default 'Mail' app easily by doing so. It's straightforward once you click on it.
Sharing a document via Outlook
The outlook app from the Microsoft Store initially did not offer the ability to share to and fro from other Microsoft Store apps. A new companion app add on called Microsoft Office Outlook Desktop Integration, which can be found here: https://www.microsoft.com/en-us/p/microsoft-office-outlook-desktop-integration/9mz70gf6gnv6 solves it.
After which you should be able to see it within the 'share' function.