NIH grant applications require you to include the PubMed Central ID number (PMCID) of any article you cite if it has one, in order to follow their public access policy. So how to you cite these using Zotero?
Zotero will automatically grab the PMCID if the article has one. You'll find it saved in the field titled "Extras," alongside the PubMed ID (PMID), if the article has one of those.
To find out which articles in your Zotero library have PMCIDs, use the search box at the top of the middle panel of your Zotero library to search for "PMCID". All the articles that appear in the center panel will have them. You can click on an article and scroll down to the "Extra" field to view it.
Zotero does not come pre-loaded with a citation style that includes PMCIDs. However, there is currently one in the Zotero Style Repository which you can add: Follow this link to the National Library of Medicine (grant proposals with PMIDs/PMCIDs) citation style. Click on the hyperlinked blue text with that title, and you will be prompted to confirm adding the style to your Zotero program. Click OK.
You will now see that citation style in your Zotero Word plugin or Google Docs plugin. When you cite with this style, you will see PMCIDs in your bibliography. See the Introduction to Zotero guide for more details on how to cite with Zotero.
If you prefer not to use this citation style, you can edit each bibliography entry manually to add the PMCID.