Please provide a more meaningful description, best including screenshots to demonstrate which settings you chose on the »Import PDF«-dialog for example.
Why do you convert the pdf, which may contain vector- and type-data after all, to a pixel image?
px is a problematic unit of measurement as it can be affected by resolution.
You cannot create a PDF with a size in pixels from Illustrator EVEN THOUGH YOU CAN CHOOSE OPTIONS THAT LOOK LIKE IT.
What size and resolution do you want in Photoshop? Or do you want it the same size ON SCREEN?