Create multipart page in onenote with PDF attachment, inserting binary data

Question

I am creating pages in onenote, using the Microsoft Graph API, which works fine with plain HTML.

To create a multipart page like here Create Onenote page with MS Graph API, the API expects binary data. I wanted to attach a multi page PDF file, but how do I get the binary data from it. Just opening with a texteditor end copying the plain text does not work.