What I have done in the past - and it's only of very limited value in some specific circumstances - is to display the part of the PDF document that I want to display in Keynote using a PDF viewer and then take a screenshot of it ('Home' and 'Power' button pressed briefly at the same time). The resulting shot ends up in Photos, from where it can be imported into Keynote.
Yes - of course - there are *lots* of limitations but for some very specific circumstances it can work - and, basically, I couldn't think of another way....
Tim
Scotland