3D FlipBook - Individual PDFs do not appear in the category under the WordPress menu

I created a category. When uploading a PDF, I assigned it to the category. It appears in the backend, but when the category is integrated as a menu item, no PDF is displayed on the corresponding website.

If I control the PDF directly as a flipbook in the menu, it works.

How can I create a menu item in WordPress on whose dependent page the PDFs of a certain category are displayed?