More and more open access (OA) journals are providing articles for free over the web. Since these are published originally online, they will not have page numbers.
They might, however, be assigned an article number. When viewing these articles in PDF, pagination is likely to start with page 1.
MLA
Omit the article number and the pagination that appears anew for each article. For example:
Bjork, Bo-Christer, and David Solomon. "Open Access versus Subscription Journals: A Comparison of Scientific Impact." BMC Medicine, vol. 10, 17 July 2012. https://doi.org/10.1186/1741-7015-10-73.
[Notice that Article number, 73, appears in the DOI.]
APA
Use the article number by including the word "Article" and then the article number instead of the page range. For example:
Bjork, B., & Solomon, D. (2012). Open access versus subscription journals: A comparison of scientific impact. BMC Medicine, 10, Article 73. https://doi.org/10.1186/1741-7015-10-73.
CSE
Use the article number in place of traditional page numbers. For example:
Bjork B, Solomon D. 2012. Open access versus subscription journals: a comparison of scientific impact. BMC Medicine. [accessed 2023 Mar 1]; 10: 73. https://doi.org/10.1186/1741-7015-10-73.