A problem of this nature often depends on the printer driver, since the driver is at least partly involved in the PDF creation process. Which printer do you have selected, and where did its driver come from (Apple via Software Update/App Store, directly from the manufacturer's web site, or was it already installed before you upgraded to Mountain Lion)?
A possible workaround might be to manually add a different brand/model of printer and select that before the print to PDF command.
BTW how do you add pages to original pdf, in SL it was on the sidebar but I dont see it anymore
Not sure that what you mean is what I understand you mean! I can open two pdfs in preview, show their thumbnails, and drag pages from one doc to another.
If you mean something else, please let me know