The question of citing e-resources can be confusing because page numbers on those resources that are not pdf "scans" of actual paper resources vary according to device used, font and type-size selection and even orientation. A quick google search yields with many offerings, including APA and MLA standards.
Listed below are academic sites found in a quick search, but the gist seems to be that without page numbers, one should list the work, author, version of text, and the chapter and paragraph number of the referred text.
One should note that most etexts (of paper-published texts) being published now include "equivalencies" for what page the cited text would be found on in the paper edition. This is true for newer Kindle books as well.
Valencia College MLA Style for Electronic Sources
Valencia College APA Style for Electronic Resources
Purdue Online Writing Lab (OWL)
MLA 5.7.18 defines digital files as neither on the web or a published CD-ROM. Digital files include a PDF on your computer, a word processed document created by a peer, a scanned image you received via e-mail, or "a sound recording formatted for playing on a digital audio player.”
To cite an electronic version of a print book on an e-book reader (APA 7.02), choose "Book" as the citation type, then select: