There is no explicit way to indicate that you are citing a PDF, as PDF is a file format and not a type of reference (e.g., journal article, book, or book chapter). You can cite any reference type in PDF format (e.g., magazine articles and reports). To cite a PDF in APA format, cite it in a webpage style. Include a URL, which takes the user to the PDF. Following are a template and example on how to cite a PDF in APA format.
Template
Surname, F. M. or Name of Group. (Publication Date). Title of the report. Publisher (if different from author). URL
Example
California Air Resources Board [CARB]. (2017). California’s 2017 climate change scoping plan. https://ww3.arb.ca.gov/cc/scopingplan/scoping_plan_2017.pdf