If it is mandatory and you want them to see it before they book, you'd just add the text to the Description> 'other things to note' section of your listing. You could also include it in your house manual.
My other answer is not wrong but here is a better one. I would put the text of your PDF on the AirBnB site under booking>house rules> additional rules. AFAIK AirBnB never allowed files to be downloaded by prospective guests prior to booking, and even after booking the only way for a guest to see a PDF would be to email it. I might be thinking of a different platform, I think some do allow files to be downloaded.