Navigate to the
“Document Library Settings” –> “Advanced Settings” –> Check the “Allow management of content types?” radio button to yes and say “OK”
Get back to the settings page..now you will be able to see a new section as “Content Types” Click on the Default content type (or the content type you need to modify).
It will list down all the columns(in the document library)…Select your require column and it will provide you with the following options.
Required (Must contain information)
Optional (May contain information)
Hidden (Will not appear in forms)
Select the Required option to make your column to mandatory contain information.