How to avoid instance name in the Document URLs in Salesforce?

Make sure My Domain is enabled in your org.

Check the below URL, if My Domain is not enabled.

http://www.infallibletechie.com/2013/04/how-to-register-domain-instead-of.html

Image - https://mydomain--c.documentforce.com/servlet/servlet.ImageServer?id={id}&oid={OrgId}

PDF - https://mydomain--c.documentforce.com/servlet/servlet.FileDownload?file={id}