Here is BDK America's recommended format for citing the works provided in dBET PDF files. The citation starts with the usual bibliographic data, followed by the URL and date of access.
Jamie Hubbard, trans. Expository Commentary on the Vimalakīrti Sutra. Berkeley: BDK America, 2012. dBET_T2186_ExpositoryCommentaryonVimalakirti_2012_0.pdf (accessed Jan.22, 2018).
Rolf W. Giebel, trans. Two Esoteric Sutras: The Adamantine Pinnacle Sutra and The Susiddhikara Sutra. Berkeley, Numata Center for Buddhist Translation and Research, 2001. dBET_TwoEsotericSutras_2001.pdf (accessed Jan. 22, 2018).
Year of Publication. Please use the year of publication as shown in the PDF of the full title page and the following copyright page.
Disambiguation. In citations, do not use the year as shown in the brief copyright notice for the dBET PDF, typically listed on the first page in the PDF. The PDF copyright is distinguished by the short format as follows: "dBET PDF Version © 2016."
Pagination. Please be advised that for filenames begining with the word "Beta," page numbers will not often correspond to the printed volume due to reformatting. Therefore if you are working from a Beta file version, please use the beta file pagination and reference the filename in your citation as shown above.