The following guide will walk through how to attach an file on SharePoint to a email on the webmail version of Outlook. You can use this method for sharing files for the Outlook Desktop client as well.
1. Find the file you need to attach on SharePoint.
2. Select the file, and then select Copy Link.
Or you can also select the file, then select the three vertical dots. Then select Copy Link.
3. After clicking Copy Link, a confirmation window will appear. Click on the bottom selection to open Link Settings.
The Link Settings give you control over how people can access the file. Read about them in more detail here: Link Settings.
4. Paste the link into the body of your email.
Learn more on the Patriot Knowledge Center: Microsoft SharePoint