Image imports in a different color space, and resamples after saving.

@ScottHaugaard 

PDF can index colour an image to save file size if it has less than 256 unique colours. PDF allows for indexing of grayscale, however, Photoshop does not so this may be why it is opening as RGB. I'd need to see the PDF to comment further, or at least screenshots from the PitStop Pro Inspector panel's pertinent tabs.

If JPEG is applied to the image, then the compression when returning from the edit in Photoshop can indeed be terrible.

You can use PitStop to remove compression, edit in Photoshop which should return a "clean" image, then you can manually apply compression to the image again in PitStop.

Roundtrips out/into the PDF can be challenging.

In a PDF, all images are treated as vector objects with a certain physical size, but no resolution. Pixel size is ignored.

What that means in practice, is that you need to set the resolution when opening into Photoshop, and if this does not match the image's original resolution, then resampling happens.

If this is for print, a fair assumption is that the original ppi is 300.

There is no reason a black and white image cannot be an RGB file. That's probably the case here. Using Image Mode to convert to grayscale is risky, because it will use whatever grayscale profile is set up in Color Settings. By default this is Dot Gain 15%, which is really an outdated profile that has a very different tone curve from what is in any practical use today. That can cause a lot of practical problems if you're not aware of it.

If you really need to convert to grayscale, use Edit > Convert to Profile, and set the target grayscale profile to Gray Gamma 2.2. That's usually pretty safe - unless it's going to offset print. That requires a more elaborate procedure (come back if that's the case).