Hi,
Are you asking about thumbnail in dam folder where pdf is added?
If so currently pdf/image is using the below rendition got created while uploading the file to dam as thumbnail
jcr:content/renditions/cq5dam.thumbnail.48.48.png
If you need custom thumbnail for pdf, I think you can write custom step and add it to DAM Update Asset workflow which will replace the above thumbnail with your custom thumbnail for pdf files.
Hope this helps!