PDF Upload Generate and Handle Thumbnail

Hello together.

I hope to get help for the following topic: Since version 4.7 WordPress generates thumbnails of the first page when I upload a PDF file with the media library. It seems I do not understand exactly how this feature works.

I use a self-written 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 with images perfectly.

When I upload a PDF, WordPress creates only the default images (thumbnail, medium, full) but not my custom image size.
So, I have two questions:

1. 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!