Enable Attach Articles Inline in the Case Feed view

Enable Case Feed Actions and Feed Items

In Salesforce Classic: Setup | Customize | Cases | Support Settings

In Lightning Experience: Gear icon | Setup | Feature Settings | Service | Support Settings

1. Click Edit.
2. Select Enable Case Feed Actions and Feed Items.
3. Click Save.

Enable Attaching Articles Inline

In Salesforce Classic: Setup | Customize | Cases | Page Layouts

In Lightning Experience: Gear icon | Setup | Object and Fields | Object Manager | Case | Case Page Layouts

1. Click Edit.
2. Click Feed View.
3. Under "Articles Tool Settings," select Enable attaching Articles inline.
4. Click Save.

Note: If the "Feed View" option is not available on the Standard or Custom Case Page Layout, you must first convert the page layout to a Feed-based case layout before this setting appears.