Hi,
Thank you for the details, it helped a lot.
The issue was, that the attachment search was turned off, I turned it on under the General Options -> Attachments search panel: https://i.imgur.com/w4bfxq7.png
I am seeing the PDF file in the results now.
The iframe extraction is a highly experimental feature, and as it is stated it may not work in all cases. On that page, there is an iframe with a custom flipbook script, embedded with a PDF reader of some sort. I’m afraid there is no way of indexing that, as it is also embedded in the iframe. That feature works great with HTML text content, it can extract most of that, but complex data – like embeds are not possible to fetch.