It is more difficult to attach a file that resides in OneDrive to a Web Email or even an Outlook Email for iOS message.
It is easier to share the document from within the OneDrive app on the iOS device.
Open OneDrive, choose the document you want to share.
In the upper right in OneDrive, you will see an icon that looks like a semi-square with an up arrow, select that
Then Select the Outlook icon and enter your recipient's email, and then Send
The recipient will receive and email that you are sharing the document with them. You may also receive an email that you Shared said document.
Note: This process works great for sharing with people inside the organization. To get the document to an outside email, a traditional attachment in Outlook.