In case you missed my Acrobat post above, AcrobatPro lets you make conversions into a single RGB space and protect CMYK black, which is what @ozdefy12345678 is trying to do.
For mixed profiles to work the print driver needs to convert from the source RGB profiles, not ignore them and assign some default RGB space. If that is happening embedded ProPhotoRGB would indeed create big output problems.
The issue with ProPhoto in this workflow is also the the limit of PDF to be supporting only 8-bit, if I rememeber correctly. For those that do use a ProPhoto in PDF workflow, please confirm that I'm not spreading rumors. I know exporting a 16-bit Photoshop file to PDFx4 will give an 8 bit image in the PDF.