Assuming the file will eventually be output from Photoshop, it will only postpone the problem, not solve it.
Images in a PDF aren't treated as pixel images, they are treated as objects at a certain size. You set the resolution (ppi) when opening into Photoshop, and unless you happen to match the original resolution, chances are you will get a resampled image.
In short, PDF images are hit or miss. I avoid them if at all possible. But to make the best out of it, open at 300 ppi in Photoshop.