PDF Upload Handle Thumbnails

Hello folks.
I’m not sure if there is a bug or I need some help.
Since version 4.7 WordPress core generates thumbnails from the first page of a PDF file within the media library.
I wrote a theme and thumbnail support is enabled. With add_image_size I added a custom image size of 300 x 300 Pixel, cropped, which is working as expected, with images.
But, when I upload a PDF, WordPress creates only the default images (which is set as thumbnail, medium, full) but not my custom image size.
So, I have two questions:

1. Is this a bug or can somebody explain me how I get WordPress done to create also the custom images sizes of a PDF?
2. Is there a way to prevent WordPress to create the default image sizes, because I do not need them?

I would really appreciate if somebody can point me to the right direction.

Thank you in advance!