I believe he refers to the resource that PDF files allow for you to make a link on the PDF document, but instead to point it to a web address or local file, store the file (byte array perhaps) in the PDF it self.
I got PDFs (not generated by PdfSharp) that have embeded resources so when you click a link it "saves" (or opens) the refered file reading it from assets stored on the very document.
I had that doubt too, and if it's possible to do it, i'd like to know how as well
Refers to
https://www.adobe.com/content/dam/acom/en/devnet/acrobat/pdfs/pdf_reference_1-7.pdf Section 3.10.3 and in note 94 in Appendix H