I see. Are these flipbooks from the dflip post type in any way related to these PDFs or you had to add them manually somehow?
Because if the files are uploaded to the post type itself, then you can choose the attachment page or the attachment parent post as the source to see if that makes any difference: https://i.imgur.com/LhLVNjs.png
Coding this is not going to be simple unless you have very good experience with custom coding. The search results URLs have to be changed and directed to a page where their recommended custom code is present, then the search results URLs changed according to that.