For the record this epubcheck issue concerns manifest fallbacks (now discouraged according to that discussion) plus mentions pdf and how best to follow the html spec.
https://github.com/w3c/epub-specs/issues/1464
Interesting discussion.
So viewing of pdf may or may not come in a future epub spec if they fully deprecate manifest fallbacks.
I will explore how best to more fully support pdf in Sigil master for a future release just in case the spec moves that way.