There was a thread in the German part of the forum:
The gist of it: pdf is a proprietary format that is not part of the mp3 standard.
So the idea is to transform the booklet into something that is supported by the standard - which is either text or picture.
E.g. you could copy the text from the pdf to a tag field, e.g. UNSYNCEDLYRICS if you want to be able to see the text in better players. In most cases the formatting of texts is not really relevant, so you could use any other plain text field.
If the layout is important, then transform the PDF into jpg files (the professional Acrobat has such a function) and add it as booklet picture.
Perhaps other users have other ideas.