Most PDFs have permanent URLS, meaning you can reference them essentially as websites by including a direct link to the document in the reference list entry. For example:
Author, A. or Corporate/Group Author. (Date). Title of PDF in italic sentence case. Title of webpage/PDF Publisher. URL
Bates, M. (2012). North America’s nearly-forgotten native vegetable. University of Missouri. http://extension.missouri.edu/platte/documents/horticulture/fruit_veg/JerusalemArtichoke.pdf
However, some PDFs can only be viewed after downloading. In these cases, your reference list entry should include the URL for the webpage from which the PDF was downloaded. Include [PDF] in square brackets after the title to indicate to the reader to download the relevant document from the site. For example:
Author, A. or Corporate/Group Author. (Date). Title of PDF in italic sentence case [PDF]. URL
Nursing Council of New Zealand. (2012). Code of conduct for nurses [PDF]. https://www.nursingcouncil.org.nz/Public/Nursing/Code_of_Conduct/NCNZ/nursing-section/Code_of_Conduct.aspx?hkey=7fe9d496-9c08-4004-8397-d98bd774ef1b