If your Visual Studio debugger says this to you, it either means
1. It gets expensive in terms of testing and (sometimes) development to read every single old PDB format, so with each release the C++ team determine what is the oldest PDB they can read is. 8.0 will read PDBs created with 5.0, but I don't believe it can read a 4.0 PDB any more.
2. A common case for this is a PDB created with 8.0, then read with Visual Studio 7.1 or earlier. There was a lack of forethought on error codes, so there was one error that mean basically "huh? can't read this".
How do you tell the version of a PDB you may ask? Believe it or not you TYPE the pdb itself: if it says
Microsoft C/C++ MSF 7.00
then it is 7.0 or newer. If it says something else, then it is older than that. Unfortunately I am not aware of any public tools that will tell you the version with greater granularity.