I'm working in several CMYK .PSD documents. Whenever I save them as .PDF, the first PDF generated always appears as a more rich RGB-looking black (on-screen at least). Changing maybe a few non-color-space/mode items and re-saving always results in a PDF with lighter looking blacks on screen. Why is this? It's driving me mad!
Photoshop won't change settings invisibly unless something is corrupted, so first make very sure that you are looking at these PDFs in Adobe Reader or Acrobat, not some other viewer.
Be sure you are not changing any settings in the PDF export dialog, nor any color settings in the Photoshop document when you open it. All your settings should be "Preserve Embedded Profile" for safety.
If all those points are okay, and you still see different blacks on consecutive saves, Photoshop may have a corrupted preferences file. Reset the preferences (hold down Ctl-Alt-Shift or Cmd--Option-Shift while the program is starting, and answer yes when it asks if you want to erase your settings).
It may just be (and this is what I think is happening) that you are running into a problem of CMYK colors in the PDF not being correctly displayed. The good news is that's not a problem with Photoshop, but with whatever you are using to view the PDF. One of the two common error situations where black "looks gray" is when CMYK values are being incorrectly translated to RGB for viewing on a monitor -- which I think may be happening in your situation.
Here's how to check:
There is quite a lot to know about saving Photoshop files (or any files) to the PDF format. In the professional world of print production, it becomes very important to know what you're doing.
[Side Note: A "Photoshop PDF" is just a PDF with a Photoshop file hidden inside. A PDF viewer or other PDF-aware program "sees" only the PDF information, but Photoshop recognizes the embedded Photoshop document and opens that part for editing. An "Illustrator PDF" works the same way. Inside the PDF is a complete native Illustrator document, hidden from view until you open the file in Illustrator.]
Photoshop PDF documents store two copies of the image they contain. The first copy is stored according to the PDF standard and uses the compression filter specified during saving, and this compression filter may be lossy such as JPEG. The second copy is stored in the additional private Photoshop data and is compressed with the lossless ZIP compression.....
as much as you'll resave the document it will reduce in color quality