Are Buddhist sutras considered "apaurusheya"?

[Matthew Kapstein]

This raises some interesting, knotty questions in fact.
You should know that "apaurusheya" means that the Vedas
are not even authored by the gods -- purushas are all
individual beings, not just humans. In general, Buddhists
found this view of scriptural authority to be objectionable,
and in works such as Santarakshita's Tattvasamgraha very
detailed philosophical refutations of the apaurusheyatvam of
the Vedas are given. Buddhist scriptures, whether the early suttas,
Mahayana sutras, or tantras, were, by contrast, generally
given careful ascription to divine or human authors, by means
of the five "ascertainments" of speaker, audience, place, time
and teaching. Nevertheless, some of the metaphysical
speculations associated with the theory of Buddha-nature
seem to countenance spontaneous eruptions of the Dharma,
authored by no discernible person, except perhaps the dharmakaya.
Some studies that discuss at least some of these issues include
Ronald Davidson's contribution to R. Buswell, Chinese Buddhist
Apocrypha
Janet Gyatso's "Letter Magic" in her edited vol. In the Mirror of Memory
and my "The Purificatory Gem and Its Cleansing" in
my book The Tibetan Assimilation of Buddhism

Matthew Kapstein