To set a static document ID within your generated documents, you can define a custom template variable (ex. DocId) within advanced settings at the organization level > under the Document configuration section. 

Within the advanced organizational settings, users can also define where this variable lives within the system documents through the 'word document' and 'excel spreadsheet' advanced settings (example below), within the Document configuration section.

Through this configuration, the option to 'modify template variables' becomes available on the release document page, by clicking on the ... and then 'modifying template variable'. This document ID will remain static across versions. This setting is supports template variable configuration of both system and templated documents.

An alternative to setting document variables [DocIDs], is renaming the documents. This option is also available to users by clicking on the ... and then 'Rename document'.