Why do DOIs and URLs show/don't show in my reference list?
The display of DOIs and URLs is usually determined by the chosen citation style. In print articles, however, these online identifiers are purposefully excluded - reference types Journal article and News article with any value set in the Pages field will not include DOIs or URLs.
The option Always include DOI and URLs can override this behavior in both Google Docs and Word, showing DOIs as specified by the citation style.
You can store multiple URLs in the field URLs. Paperpile will always use the first of the list in citations.
Some citation styles don't suport displaying DOIs and this option (despite its name) cannot change that. You may need to adapt the behavior of the citation style by editing the CSL file.