<p>I have a Visual Studio (2008) solution consisting of several projects, not all in the same namespace. When I build the solution, all the DLL files used by the top level project, <strong>TopProject</strong>, are copied into the <em>TopProject\bin\debug</em> folder. However, the corresponding .pdb files are only copied for some of the other projects. This is a pain, for example when using <a href="http://www.ndepend.com/" rel="nofollow noreferrer">NDepend</a>.</p>

<p>How does Visual Studio decide which .pdb files to copy into higher level bin\debug folders? How can I get Visual Studio to copy the others too?</p>

<hr>

<p>References are as follows: all the DLL files are copied to a central location, without their PDB files. <strong>TopProject</strong> <em>only</em> has references to these copied DLL files; the DLL files themselves, however, evidently know where their PDB files are, and (most of them) get copied to the <code>debug</code> folder correctly.</p>

From
MSDN
:
A program database (PDB) file holds
  debugging and project state
  information that allows incremental
  linking of a Debug configuration of
  your program. A PDB file is created
  when you compile a C/C++ program with
  /ZI or /Zi or a Visual
  Basic/C#/JScript .NET program with
  /debug.
So it looks like the "issue" here (for lack of a better word) is that some of your DLLs are being built in debug mode (and hence emitting PDB files), and some are being built in release mode (hence not emitting PDB files). If that's the case, it should be easy to fix -- go into each project and update its build settings. This would be the default scenario, if you haven't done any tweaking of command line options.
However, it will get trickier if that isn't the case. Maybe you're all in release or debug mode. Now you need to look at the command line compile options (specified in the project properties) for each project. Update them to /debug accordingly if you want the debugger, or remove it if you don't.
Edit in Response to Edit
Yes, the DLL files "know" that they have PDB files, and have paths to them, but that doesn't mean too much. Copying just DLL files to a given directory, as others have mentioned, won't clear this issue up. You need the PDB files as well.
Copying individual files in Windows, with the exception of certain "bundle"-type files (I don't know Microsoft's term for this, but "complete HTML packages" are the concept) doesn't copy associated files. DLL files aren't assembled in the "bundle" way, so copying them leaves their PDB file behind.
I'd say the only answer you're going to have is to update your process for getting the DLL files to those central locations, and include the PDB files ... I'd love to be proven wrong on that, though!