The preview images for PDF documents are created within wp_generate_attachment_metadata() [src].
The full size preview image will have the -pdf.jpg extension [src].
The preview images info for the PDF document is stored in _wp_attachment_metadata in the post meta table.
Here’s an example data for a test.pdf document:
Array
(
[0] => Array
(
[sizes] => Array
(
[thumbnail] => Array
(
[file] => test-pdf-116x150.jpg
[width] => 116
[height] => 150
[mime-type] => image/jpeg
)
[full] => Array
(
[file] => test-pdf.jpg
[width] => 1088
[height] => 1408
[mime-type] => image/jpeg
)
)
)
)
This is stored as a serialized array under the _wp_attachment_metadata key in the post meta table, but we display it here as an array for better readability.
It’s fetched with wp_get_attachment_metadata() [src], as for any other image attachment.
We can see it e.g. in wp_prepare_attachment_for_js() [src].
Also in image_downsize()[src], that’s used by wp_get_attachment_image_src() [src].
The latter one is used by wp_get_attachment_image(), that’s called within the column_title() method of the media list table, to display the image preview [src].
Hope it helps!