Currently, you can’t embed presentations, PDFs or spreadsheets to your case study.
That doesn’t mean you can’t use your materials to enrich your case studies. You can link your original files to your case studies as sources, so people can open if they are interested in more details:
Upload your files to your favourite cloud storage (like Google Drive or Dropbox)
Copy the link of your file.
Select a text in your case study.
Click the link button in the toolbar.
Paste your link into the box.