EDIT: If it's important that it's the actual standardized version rather than the final draft then you can get it for free here: https://www.pdfa-inc.org/product/iso-32000-2-pdf-2-0-bundle-...
(Strictly speaking it's the 2020 revision which incorporates some errata. I don't know if the final standardized 2017 version was made freely available)
EDIT: If it's important that it's the actual standardized version rather than the final draft then you can get it for free here: https://www.pdfa-inc.org/product/iso-32000-2-pdf-2-0-bundle-...
(Strictly speaking it's the 2020 revision which incorporates some errata. I don't know if the final standardized 2017 version was made freely available)