Thank you, davidfor, I will try that.
Sirtel,
I contacted KDP and they verified that it is, indeed, a MOBI file. As I mentioned, I knew it was a Mobi file when I uploaded it and thought it was strange for them to convert it back to a PDF before publishing. As mentioned, the conversion was done using
Kindle Kids Book Creator:
https://kdp.amazon.com/en_US/help/topic/G201562880
The folks at KDP are perplexed as to what is making you think it is a PDF?
Nevertheless, for anyone following this thread who is experiences similar issues, I'm hoping much of this info will be useful.