After a DOI is assigned to an object, Crossref requires that DOI links be included in that object's reference list.
DOIs in the reference lists can appear as a Crossref DOI URL, ShortDOI, or hyperlinked text “Crossref.” For more information on these options, visit http://www.crossref.org/02publishers/doi_display_guidelines.html
Ideally, DOIs will be added to reference lists by authors at the time of publication. However, this is not always the case. Uploading supplementary documents of reference lists is a solution for articles originally published with reference lists without DOIs.
Crossref provides tools to help you find DOIs for citations in reference lists. For more information of these tools, visit https://support.crossref.org/hc/en-us/articles/215785803-Reference-linking