Zotero is an easy to use application to help you collect, organize, cite and share your research sources. Zotero can also format your citations according to a APA style and make in-text citations in Word easier.
A DOI is a Digital Object Identifier.
It is a unique alphanumeric string assigned by a registration agency (the International DOI Foundation) to identify content and provide a persistent link to its location on the Internet. The publisher assigns a DOI when your article is published and made available electronically.
All DOI numbers begin with a 10 and contain a prefix and a suffix separated by a slash. The prefix is a unique number of four or more digits assigned to organizations; the suffix is assigned by the publisher and was designed to be flexible with publisher identification standards.
The APA Publication Manual 7th edition requires you to include a DOI for all works that have a DOI.
American Psychological Association. (2019). DOIs and URLs. Retrieved from https://apastyle.apa.org/style-grammar-guidelines/references/dois-urls
Usually the DOI can be found on the first page of the electronic journal article or if the article is found in a database, in the bibliographic record in the database.
If you don't see one and want to check, you can go to crossref.org and type in the author's last name and the name of the journal article.
If a DOI is available, you will need to add it to your citation.
Karplus, M. (2010). Dynamical aspects of molecular recognition. Journal of Molecular Recognition, 23(2), 102-104. https://doi.org/10.1002/jmr.1018
For more information about APA Format, you can check out the APA Manual or the APA Style Blog.